Hi ■:W-?. ^^..^Mj.^,.T,.^ .J > . n T'; 44r^^ TWENTIETH CENTURY TEXT-BOOKS EDITED BY A. F. NIGHTINGALE, Ph.D., LL.D. SUPERINTENDENT OF SCHOOLS, COOK COUNTY, ILLINOIS TWENTIETH CENTURY TEXT-BOOKS PLANT STRUCTURES A SECOND BOOK OF BOTANY BY JOHN M. COULTER, A.M., Ph.D. HEAD OF DEPARTMENT OF BOTANY UNIVERSITY OF CHICAGO SECOND EDITION REVISED NEW YORK D. APPLETON AND COMPANY 1904 Copyright, 1899, 1904, By D. APPLETON AND COHIPANY. PKEFACE In the preface to Plant Relations the author gave his reasons for suggesting that the ecological standpoint is best adapted for the first contact with plants. It may be, how- ever, that many teachers will prefer to begin with the mor- phological standpoint, as given in the present book. Rec- ognizing this fact, Plant Structures has been made an independent volume that may precede or follow the other, or may provide a brief course of botanical study in itself. Although in the present volume Morphology is the domi- nant subject, it seems wise to give a somewhat general view of plants, and therefore Physiology, Ecology, and Taxonomy are included in a general way. For fear that Physiology and Ecology may be lost sight of as distinct subjects, and to introduce important topics not included in the body of the work, short chapters are devoted to them, which seek to bring together the main facts, and to call attention to the larger fields. This book is not a laboratory guide, but is for reading and study in connection with laboratory ivorTc. An accom- panying pamphlet for teachers gives helpful suggestions to those who are not already familiar with its scope and purpose. It is not expected that all the forms and sub- jects presented, in the text can be included in the labora- tory exercises^ but it is believed that the book will prove a useful companion in connection with such exercises. It is very necessary to co-ordinate the results of laboratory work, to refer to a larger range of material than can be handled, and to develop some philosophical conception of yi PREFACE the plant kingdom. The learning of methods and the collection of facts are fundamental processes, but they must be supplemented by information and ideas to be of most service. The author does not believe in the use of technical terms unless absolutely necessary, for they lead frequently to mistaking definitions of words for knowledge of things. But it is necessary to introduce the student not merely to the main facts but also to the literature of botany. Ac- cordingly, the most commonly used technical terms are introduced, often two or three for the same thing, but it is hoped that familiarity with them will enable the student to read any ordinary botanical text. Care has been taken to give definitions and derivations, and to call repeated attention to synonymous terms, so that there may be no confusion. The chaotic state of morphological terminology tempted the author to formulate or accept some consistent scheme of terms ; but it was felt that this would impose upon the student too great difficulty in reading far more important current texts. Chapters I-XII form a connected whole, presenting the general story of the evolution of plants from the lowest to the highest. The remaining chapters are supplementary, and can be used as time or inclination permits, but it is the judgment of the author that they should be included if possible. The flower is so conspicuous and important a feature in connection with the highest plants, that Chapter XlII seems to be a fitting sequel to the preceding chapters. It also seems desirable to develop some knowledge of the great Angiosperm families, as presented in Chapter XIV, since they are the most conspicuous members of every flora. In this connection, the author has been in the habit of directing the examination of characteristic flowers, and of teaching the use of ordinary taxonomic manuals. Chap- ter XV deals with anatomical matters, but the structures included are so bound up with the form and work of plants PREFACE yji that it seems important to find a place for them even in an elementary work. The reason for Chapters XVI and XVII has been stated already, and even if Plant Relations is stud- ied, Chapter XVII will be useful either as a review or as an introduction. In the chapter on Plant ' Physiology the author has been guided by Xoll's excellent resume in the " Strasburger " Botany. The illustrations have been entirely in the charge of Dr. Otis W. Caldwell, who for several years has conducted in the University, and in a most efficient way, such labo- ratory work as this volume implies. Many original illus- trations have been prepared by him, and under his direction by Messrs. S. M. Coulter, B. A, Goldberger, W. J. G. Land, and A. 0. Moore, and some are credited to Dr. Chamberlain and Dr. Cowles, of the University, but it is a matter of regret that pressure of work and time limitation have for- bidden a still greater number. The authors of the original illustrations are cited, and where illustrations have been obtained elsewhere the sources are indicated. The author would again call attention to the fact that this book is merely intended to serve as a compact supple- ment to three far more important factors : the teacher, the laboratory, a?id field work. John M. Coulter. TuE UxivERSiTY OF CHICAGO, November, 1899. PREFACE TO THE REVISED EDITI0:N^ During the last five years the science of Botany has made rapid progress, both in the addition of new facts and in changed points of view. Some of this progress affects Plant Structures^ and it is recorded in this revised edition so far as it can be without a complete rewriting of the volume. Changes will be found, therefore, in statements of fact, in points of view, in terminology, in illustrations, and also in the addition of new material. John M. Coulter. Tns University of Chicago, April, 1904. CONTENTS CHAPTER PAGE I. — Introduction = . . 1 II. — Thallophytes : Alg^ 4 III. — The evolution of sex 13 IV. — The great groups of Alg^ ... . . 17 V. — Thallophytes: Fungi 48 VI. — The food of plants 83 VII. — Bryophytes 93 VIII. — The great groups of Bryophytes 109 IX. — Pteridophytes 128 X. — The great groups of Pteridophytes .... 155 XI. — Spermatophytes : Gymnosperms 171 XII. — Spermatophytes : Angiosperms 195 XIII. — The flower 218 XIV. — Monocotyledons and dicotyledons 232 XV. — Differentiation of tissues 280 XVI. — Plant physiology 297 XVII, — Plant ecology 311 Glossary 329 Index 337 ix BOTANY PART II.— PLANT STRUCTURES CHAPTER I INTRODUCTION 1. Differences in structure. — It is evident, even to the casual observer, that plants differ very much in structure. They differ not merely in form and size, but also in com- plexity. Some plants are simple, others are complex, and the former are regarded as of lower rank. Beginning with the simplest plants — that is, those of lowest rank — one can pass by almost insensible grada- tions to those of highest rank. At certain points in this advance notable interruptions of the continuity are dis- covered, structures, and hence certain habits of work, chang- ing decidedly, and these breaks enable one to organize the vast array of plants into groups. Some of the breaks ap- pear to be more important than others, and opinions may differ as to those of chief importance, but it is customary to select three of them as indicating the division of the plant kingdom into four great groups. 2. The great groups. — The four great groups may be indicated here, but it must be remembered that their names mean nothing until plants representing them have been studied. It will be noticed that all the names have the 1 2 PLANT STKUCTURES constant termination phytes^ which is a Greek word mean- ing " plants." The prefix in each case is also a Greek word intended to indicate the kind of plants. (1) Thallophytes. — The name means "thallus plants," but just what a " thallus " is can not well be explained until some of the plants have been examined. In this great group are included some of the simplest forms, known as Algce and Fu7igi, the former represented by green thready growths in fresh water and the great host of sea- weeds, the latter by moulds, mushrooms, etc. (2) Bryophytes.— The name means " moss plants," and suggests very definitely the forms which are included. Every one knows mosses in a general way, but associated with them in this great group are the allied liverworts, which are very common but not so generally known. (3) Pteridophytes. — The name means " fern plants," and ferns are well known. Not all Pteridophytes, however, are ferns, for associated with them are the horsetails (scouring rushes) and the club mosses. (4) Spermatophytes. — The name means " seed plants " — that is, those plants which produce seeds. In a general way these are the most familiar plants, and are commonly spoken of as " flowering plants." They are the highest in rank and the most conspicuous, and hence have received much attention. In former times the study of botany in the schools was restricted to the examination of this one group, to the entire neglect of the other three great groups. 3. Increasing complexity. — At the very outset it is well to remember that the Thallophytes contain the simplest plants — those whose bodies have developed no organs for special work, and that as one advances through higher Thallophytes, Bryophytes, and Pteridophytes, there is a con- stant increase in the complexity of the plant body, until in the Spermatophytes it becomes most highly organized, with numerous structures set apart for special work, just as in the highest animals limbs, eyes, ears, bones, muscles, nerves, etc., INTRODUCTION 3 are set apart for special work. The increasing complexity is usually spoken of as differentiatio7i — that is, the setting apart of structures for different kinds of work. Hence the Bryophytes are said to be more highly differentiated than the Thallophytes, and the Spermatophytes are regarded as the most highly differentiated group of plants. 4. Nutrition and reproduction. — However variable plants may be in complexity, they all do the same general kind of work. Increasing complexity simply means an attempt to do this work more effectively. It is plant work that makes plant structures significant, and hence in this book no at- tempt will be made to separate them. All the work of plants may be put under two heads, nutrition and repro- duction, the former including all those processes by which a plant maintains itself, the latter those processes' by which it produces new plants. In the lowest plants, these two great kinds of work, or functions, as they are called, are not set apart in different regions of the body, but usually the first step toward differentiation is to set apart the re- productive function from the nutritive, and to develop special reproductive organs which are entirely distinct from the general nutritive body. 5. The evolution of plants.— It is generally supposed that the more complex plants have descended from the simpler ones ; that the Bryophytes have been derived from the Thallo- phytes, and so on. All the groups, therefore, are supposed to be related among themselves in some way, and it is one of the great problems of botany to discover these relation- ships. This theory of the relationship of plant groups is known as the theori/ of descent, or more generally as evo- lution. To understand any higher group one must study the lower ones related to it, and therefore the attempt of this book will be to trace the evolution of the plant king- dom, by beginning with the simplest forms and noting the gradual increase in complexity until the highest forms are reached. CHAPTEE II THALLOPHYTES : ALG-Sl 6. General characters. — Thallophytes are the simplest of plants, often so small as to escape general observation, but sometimes with large bodies. They occur everywhere in large numbers, and are of special interest as representing the beginnings of the plant kingdom. In this group also there are "organized all of the principal activities of plants, so that a study of Thallophytes furnishes a clew to the structures and functions of the higher, more complex groups. The word " thallus " refers to the nutritive body, or vegetative body, as it is often called. This body does not differentiate special nutritive organs, such as the leaves and roots of higher plants, but all of its regions are alike. Its natural position also is not erect, but prone. While most Thallopliytes have thallus bodies, in some of them, as in certain marine forms, the nutritive body differentiates into regions which resemble leaves, stems, and roots ; also cer- tain Bryophytes have thallus bodies. The thallus body, therefore, is not always a distinctive mark of Thallophytes, but must be supplemented by other characters to determine the group. 7. Algae and Fungi.— It is convenient to separate Thallo- phytes into two great divisions, known as AlgcB and Fungi. It should be known that this is a very general division and not a technical one, for there are groups of Thallophytes which can not be regarded as strictly either Algte or Fungi, but for the present these groups may be included. 4 THALLOPHYTES: ALG^ 5 The great distinction between these two divisions of Thallophytes is that the Algae contain chlorophyll and the Fungi do not. Chlorophyll is the characteristic green color- ing matter found in plants, the word meaning " leaf green," It may be thought that to use this coloring material as the basis of such an important division is somewhat superficial, but it should be known that the presence of chlorophyll gives a peculiar power — one which affects the whole structure of the nutritive body and the habit of life. The presence of chlorophyll means that the plant can make its own food, can live independent of other plants and animals. Algae, therefore, are the independent Thallophytes, so far as their food is concerned, for they can manufacture it out of the inorganic materials about them. The Fungi, on the other hand, contain no chlorophyll, can not manufacture food from inorganic material, and hence must obtain it already manufactured by plants or animals. In this sense they are dependent upon other or- ganisms, and this dependence has led to great changes in structure and habit of life. It is supposed that Fungi have descended from Algae — that is, that they were once Alg^e, which gradually acquired the habit of obtaining food already manufactured, lost their chlorophyll, and became absolutely dependent and more or less modified in structure. Fungi may be regarded, there- fore, as reduced relatives of the Algfe, of equal rank so far as birth and structure go, but of very different habits. ALG^ 8. General characters. — As already defined. Algae are Thallophytes which contain chlorophyll, and are therefore able to manufacture food from inorganic material. They are known in general as " seaweeds," although there are fresh-water forms as well as marine. They are exceedingly variable in size, ranging from forms visible only by means 19 Q PLANT STRUCTURES of the compound microscope to marine forms with enor- mously bulky bodies. In general they are hydrophytes — that is, plants adapted to life in water or in very moist places. The special interest connected with the group is that it is supposed to be the ancestral group of the plant kingdom — the one from which the higher grou2)s have been more or less directly derived. In this regard they differ from the Fungi, which are not supposed to be responsible for any higher groups. 9. The subdivisions. — Although all the Alg^e contain chlorophyll, some of them do not appear green. In some of them another coloring matter is associated with the chlo- rophyll and may mask it entirely. Advantage is taken of these color associations to separate Algge into subdivisions. As these colors are accompanied by constant differences in structure and work, the distinction on the basis of colors is more real than it might appear. Upon this basis four sub- divisions may be made. The constant termination phycecB, which appears in the names, is a Greek word meaning " sea- w^eed," which is the common name for Algse ; while the pre- fix in each case is the Greek name for the color which char- acterizes the group. The four subdivisions are as follows : (1) Cyanopliycem, or " Blue Algas," but usually called " Blue-green Alga3," as the characteristic blue does not entirely mask the green, and the general tint is bluish-green ; (2) Chlorophycece, or " Green Algge," in which there is no special coloring matter associ- ated with the chlorophyll ; (3) Phceophycece., or " Brown Algae " ; and (4) Rhodojjhycece, or " Eed Algge." It should be remarked that probably the Cyanophyceas do not belong with the other groups, but it is convenient to present them in this connection. 10. The plant body. — By this phrase is meant the nutri- tive or vegetative body. There is in plants a unit of struc- ture known as the cell. The bodies of the simplest plants consist of but one cell, while the bodies of the most com- TIIALLOIMIYTES: ALG.E plex plants consist of very many cells. It is necessary to know something of the ordinary living plant cell before the bodies of Alga? or any other 2)lant bodies can be under- stood. Such a cell if free is approximately spherical in outline, (Fig. 6), but if pressed upon by contiguous cells may become variously modified in form (Fig. 1). Bounding it there is a thin, elastic wall, com- posed of a substance called cellulose. The cell wall, therefore, forms a delicate sac, which contains the liv- ing substance known as pro- toplasm. This is the sub- stance which manifests life, and is the only substance in the plant which is alive. It is the protoplasm which has organized the cellulose wall about itself, and which does all the plant work. It is a fluid substance which varies much in its consistence, sometimes being a thin vis- cous fluid, like the white of an egg, sometimes much more dense and compactly organized. The protophism of the cell is organized into various structures which are called organs of the cell, each organ having one or more special functions. One of the most conspicuous organs of the living cell is the single nucleus, a, comparatively compact and usually spherical protoplasmic body, and generally centrally placed within the cell (Fig. 1). All about the nucleus, and filling up the general cavity within the cell wall, is an organized mass of much thinner protoplasm, known as cytoplasm. The cytoplasm seems to form tlie general background or matrix of the cell, and the Fig. 1. t^-\U fruui a ui...-^ Kaf, r.l,..uu.s nucleus (B) in which there is a nucle- olus, cytoplasm (C), and chloroplasts (^4 ). — Caldwell. 8 PLANT STRUCTUKES nucleus lies imbedded within it (Fig. 1). Every working cell consists of at least cytoplasm and nucleus. Sometimes the cellulose wall is absent, and the cell then consists sim- ply of a nucleus with more or less cytoplasm organized about it, and is said to be naked. Another protoplasmic organ of the cell, very conspicuous among the Algaj and other groups, is the plastid. Plastids are relatively compact bodies, commonly spherical, variable in number, and lie imbedded in the cytoplasm. There are various kinds of plastids, the most common being the one which contains the chlorophyll and hence is stained green. The chlorophyll-containing plastid is known as the cliloro- flastid, or chloroplast (Fig. 1). An ordinary alga-cell, there- fore, consists of a cell wall, within which the protoplasm is organized into cytoplasm, nucleus, and chloroplasts. The bodies of the simplest Algge consist of one such cell, and it may be regarded as the simplest form of plant body. Starting with such forms, one direction of advance in complexity is to organize several such cells into a loose row, which resembles a chain (Fig. 4) ; in other forms the cells in a row become more compacted and flattened, form- ing a simple filament (Figs. 2, 5) ; in still other forms the original filament puts out branches like itself, producing a branching filament (Fig. 8). These filamentous bodies are very characteristic of the Algae. Starting again with the one-celled body, another line of advance is for several cells to organize in two directions, forming a plate of cells. Still another line of advance is for the cells to organize in three directions, forming a mass of cells. The bodies of Algae, therefore, may be said to be one- celled in the simplest forms, and in the most complex forms they become filaments, plates, or masses of cells. 11. Reproduction. — In addition to the work of nutrition, the plant body must organize for reproduction. Just as the nutritive body begins in the lowest forms with a single cell TIIALLOPIIYTES: ALG^ 9 and becomes more complex in the higher forms, so repro- duction begins in very simple fashion and gradnally be- comes more comjjlex. Two general types of reproduction are employed by the Algae, and all other plants. They are as follows : (1) Vegetative multiplication. — This is the only type of reproduction employed by the lowest Algas, but it persists in all higher groups even when the other method has been introduced. In this type no special reproductive bodies are formed, but the ordinary vegetative body is used for the purpose. For example, if the body consists of one cell, that cell cuts itself into two, each half grows and rounds off as a distinct cell, and two new bodies appear where there was one before (Figs. 3, 6). This process of cell division is very complicated and important, involving a division of nucleus and cytoplasm so that the new cells may be organized just as was the old one. Wherever ordinary nutritive cells are used directly to produce new plant bodies the process is vegetative vudtiplication. This method of rejiroduction may be indicated by a formula as follows : P — P — P — P — P, in which P stands for the plant, the formula indicating that a succession of plants may arise- directly from one another without the interposition of any special structure. (2) Spores. — Spores are cells which are specially organ- ized to reproduce, and are not at all concerned in the nutri- tive work of the plant. Spores are all alike in their power of reproduction, but they are formed in two very distinct ways. It must be remembered that these two tyj^es of spores are alike in power but different in origin. Asexual spores. — These cells are formed by cell divi- sion. A cell of the plant body is selected for the purpose, and usually its contents divide and form a variable number of new cells within the old one (Fig. 2, B). These new cells are asexual spores., and the cell which has formed them within itself is known as tlie mother cell. This peculiar kind of cell division, which does not involve the wall of the IQ PLANT STKUCTUKES old cell, is often called internal division, to distinguish it from fission, which involves the wall of the old cell, and is the ordinary method of cell division in nutritive cells. If the mother cell which produces the spores is different from the other cells of the plant body it is called the sporan- giu7n, which means " spore vessel." Often a cell is nutri- tive for a time and afterward becomes a mother cell, m which case it is said to function as a sporangium. The wall of a sporangium usually opens, and the spores are dis- charged, thus being free to produce new plants. Various names have been given to asexual spores to indicate certain peculiarities. As Algas are mostly surrounded by water, the characteristic asexual spore in the group is one that can swim by means of minute hair-like processes or cilia, which have the power of lashing the water (Fig. 7, C). These ciliated spores are known as zoospores, or " animal- like spores," referring to their power of locomotion ; some- times they are called sjcimming spares, or swarm spores. It must be remembered that all of these terms refer to the same thing, a swimming asexual spore. This method of reproduction may be indicated by a for- mula as follows : P — o — P — o — P — o — P, which indi- cates that new plants are not produced directly from the old ones, as in vegetative multiplication, but that between the successive generations there is the asexual spore. Sexual spores. — These cells are formed by cell union, two cells fusing together to form the spore. This process of forming a spore by the fusion of two cells is called the sexual process, and the two special cells (sexual cells) thus used are known as gametes (Fig. 2, C, d, e). It must be noticed that gametes are not spores, for they are not able alone to produce a new plant ; it is only after two of them have fused and formed a new cell, the spore, that a plant can be produced. The spore thus formed does not differ in its power from the asexual spore, but it differs very much in its method of origin. TIIALLOPIIYTES: ALG^ H The gametes are organized within a mother cell, and if this cell is distinct from the other cells of the plant it is called a gametanginm, which means " gamete vessel." This method of reproduction may be indicated by a for- mula as follows : P = °>o — P = °>o — P = ^>o — P, which indicates that two special cells (gametes) are pro- duced by the plant, that these two fuse to form one (sexual spore), which then produces a new plant. It must not be supposed that if a plant uses one of these three methods of reproduction (vegetative multiplication, asexual spores, sexual spores) it does not employ the other two. All three methods may be employed by the same plant, so that new plants may arise from it in three differ- ent#v7ays. CHAPTEK III THE EVOLUTION OF SEX 12. The general problem. — In the last chapter it was re- marked that the simplest Algae reproduce only by vegetative multiplication, the ordinary cell division (fission) of nutri- tive cells multiplying cells and hence individuals. Among other low Algfe asexual spores are added to fission as a method of reproduction, the spores being also formed by cell division, generally internal division. In higher forms gametes appear, and a new method of reproduction, the sexual, is added to the other two. Sexual reproduction is so important a process in all plants except the lowest, that it is of interest to discover how it may have originated, and how it developed into its highest form. Among the Algae the origin and develop- ment of the sexual process seems to be plainly suggested ; and as all other plant groups have probably been derived more or less directly from Algae, what has been accom- plished for the sexual process in this lowest group was probably done for the whole plant kingdom. 13. The origin of gametes. — One of the best Algae to illustrate the possible origin of gametes is a common fresh- water form known as Ulothrix (Fig. 2). The body consists of a simple filament composed of a single row of short cells (Fig. 2, A). Each cell contains a nucleus, and a single large chloroplast which has the form of a thick cyl- inder investing the rest of the cell contents. Through the microscope, if the focus is upon the center of the cell, an optical section of the cylinder is obtained, the chloro- 13 THE EVOLUTION OF SEX 13 plast appearing as a thick green mass on each side of the central nucleus. As no other color appears, it is evident that Ulothrix is one of the Chlorophyceas. Fig. 2. Ulothrix, a Conferva form. A, base of filament, showing lowest holdfast cell and five vegetative cells, each with its single conspicuous cylindrical chloro- plast (seen in section) inclosing a nucleus; B, four cells containing numerous small zoospores, the others emptied; f, fragment of a filament showing one cell (a) containing four zoospores, another zoospore {b) displaying four cilia at its pointed end and just having escaped from its cell, another cell (c) from which most of the small biciliate gametes have escaped, gametes pairing (d"), and the resulting zygotes ( Fig. 21. Sexual reproduction of Fncus, showing the eight eggs (six in sight) dis- charged from the oogonium and surrounded by a membrane (^4), eggs liberated from the membrane (E), antheridium containing sperms (C), the discharged lat- erally biciliate sperms (G), and eggs surrounded by swarming sperms {F, H).— After Singer. 21 S8 PLANT STRUCTURES The other group, represented by Fucus (Fig. 21), pro- duces no asexual spores, but is heterogamous. A single oogomum usually forms eight eggs (Fig. 21, A), which are discharged and float freely in the water (Fig. 21, E). The antheridia (Fig. 21, C) produce numerous minute laterally biciliate sperms, which are discharged (Fig. 21, G), swim in great numbers about the large eggs (Fig. 21, F^ H), and finally one fuses with an egg, and an oospore is formed. As the sperms swarm very actively about the egg and impinge against it they often set it rotating. Both an- theridia and oogonia are formed in cavities of the thallus. 4. Rhodophtce^e {Red Algce) 31. General characters. — On account of their red colora- tion these forms are often called Floridece. They are mostly marine forms, and are it anchored by holdfasts of various kinds. They belong to the deepest waters in which Algae grow, and it is probable that the red coloring matter which character- izes them is associated with the depth at which they live. The Eed Algae are also a high- ly specialized line, and will be mentioned very briefly. 32. The plant body. — The Eed Alga3, in general, are more deli- cate than the Brown Algfe, or kelps, their graceful forms, delicate texture, and brightly tinted bodies (shades of red, violet, dark purple, ?i6. 22. A red alga {Gigai-tina), showing branching habit, and "fruit bodies." — After ScHENCK. Fig. 24. A red alga (Dasya), showing a finely divided thallus body. Caldwell. Fig. 25. A red alga (Rabdonia). showiui,' luiklfa.^t.s and linmrliiiig thallus body.- Caldwell. Fig. 26. A red alga (Ptilota), whose branching body resembles moss. — Caldwell. THE GREAT GROUPS OF ALG^ 43 and reddish-brown) making them very attractive. They show the greatest variety of forms, branching filaments, ribbons, and filmy plates prevailing, sometimes branching very profusely and delicately, and resembling mosses of fine texture (Figs. 23, 23, 24, 25, 26). The differentiation of the thallus into root and stem and leaf-like structures is also common, as in the Brown Algae. 33. Reproduction. — Red Alg^e are very peculiar in both their asexual and sexual reproduction. A sporangium pro- duces just four asexual spores, but they have no cilia and no power of motion. They can not be called zoospores, therefore, and as each spo- FiG. 27. A red alga ( Callithamnion), show- ing sporangium (A), and the tetraspores discharged (iJ).— After Thuket. Fig. 28. A red alga (Nemalion) ; A, sexual branches, showing antheri- dia (a), oogonium (o) with its trich- ogyne (t), to which are attached two epermatia (s); B, beginning of a cystocarp (o), the trichogyne (0 still showing; C, an almost mature cj's- tocarp (0), with the disorganizing trichogyne (<). — After Knt. rangium always produces just four, they have been called tetraspores (Fig. 27). Eed Algaj are also heterog- amous, but the sexual process has been so much and so variously modified that it is very poorly understood. The antheridia (Fig. 28, ^, a) develop sperms which, like the tetraspores, have no cilia and no power of motion. To dis- 44 PLANT STRUCTURES tinguish them from the ciliated sperms, or spermatozoids, which have the power of locomotion, these motionless male gametes of the Eed Algse are usually called sjpermatia (singular, spermatiwn) (Fig. 28, A, s). The oogonium is very pe- culiar, being differentiated into two regions, a bulbous base and a hair-like process (trichogyne), the whole struc- ture resembling a flask with a long, narrow neck, excepting that it is closed (Fig. 28, A, 0, t). Within the bulbous part fertilization usually takes place ; a spermatium attaches itself to the trichogyne (Fig. 28, A, s); at the point of contact the two walls become perforated, and the contents of the spermatium thus enter the trichogyne, and so reach the bulbous base of the oogo- nium. The above account rep- resents the very simplest con- ditions of the process of fer- tilization in this group, and gives no idea of the great and puzzling complexity exhibited by the majority of forms. After fertilization the trich- ogyne wilts, and the bulbous base in one way or another develops a conspicuous struc- ture called the cijstocarp (Figs. 28, 29), which is a case con- taining asexual spores ; in other words, a spore case, or kind of sporangium. In the life history of a red alga, there- FiG. 29. A branch of Polyslphonia, one of the red algm, showing the rows of cells composing the body {A), small branches or hairs {B), and a cystocarp (C) with escaping spores {D) which have no cilia (car- pospores). — Caldweix. THE GREAT GROUPS OF ALG.E 45 foro, two sorts of asexual spores are produced : (1) the tetrasjmres, developed in ordinary sporangia ; and (8) the carpospores, developed in the cystocarp, which has been produced as the result of fertilization. OTHER CHLOROPHYLL-CONTAIlSriNG THALLOPHYTES 34. Diatoms. — These are peculiar one-celled forms, which occur in very great abundance in fresh and salt waters. Fig. 30. A group of Diatoms : c and d, top and side views of the same form; e, colony of stalked forms attached to an alga; /and gf, top and side views of the form shown at e; A, a colony; i, a colony, the top and side view shown at X;.— After Kerneu. They are either free-swimming or attached by gelatinous stalks; solitary, or connected in bands or chains, or im- bedded in gelatinous tubes or masses. In form they are rod-shaped, boat-shaped, elliptical, wedge-shaped, straight or curved (Fig. 30). 46 PLANT STRUCTURES The chief peculiarity is that the wall is composed of two valves, one of which fits into the other like the two parts of a pill box. This wall is so impregnated with silica that it is practically indestructible, and siliceous skeletons of dia- toms are preserved abundantly in certain rock deposits. They multiply by cell division in a peculiar way, and some of them have been observed to con- jugate. They occur in such numbers in the ocean that they form a large part of the free-swimming forms on the sur- face of the sea, and doubtless showers of the siliceous skeletons are constant- ly falling on the sea bottom. There are certain deposits known as "si- liceous earths," which are simply masses of fossil diatoms. Diatoms have been variously placed in schemes of classification. Some have put them among the Brown Algae because they contain a brown coloring matter; others have placed them in the Conjugate forms among the Green Algae on account of the occasional conjugation that has been observed. They are so different from other forms, however, that it seems best to keep them separate from all other Algae. 35. Characese. — These are common- ly called " stoneworts," and are often included as a group of Green Algae, as they seem to be Thallophytes, and have no other coloring matter than chlorophyll. However, they are so peculiar that they are better kept by themselves among the Algse. They are such Fig. 31. A common Chara, showing tip of main axis. — After Strasburger. THE GKEAT GROUPS OF ALG^ 47 specialized forms, and are so much more highly organized than all other Algae, that they will be passed over here with a bare mention. They grow in fresh or brackish waters, fixed to the bottom, and forming great masses. The cylin- drical stems are Jointed, the joints sending out circles of branches, which repeat the jointed and branching habit (Fig. 31). The walls become incrusted with a deposit of lime, which makes the plants harsh and brittle, and has sug- gested the name " stoneworts." In addition to the highly organized nutritive body, the antheridia and oogonia are peculiarly complex, being entirely unlike the simple sex organs of the other Algge. CHAPTEE V THALLOPHYTES : FUNGI 36. General characters. — In general, Fungi include- Thal- lophytes whicli do not contain chlorophyll. From this fact it follows that they can not manufacture food entirely out of inorganic material, but are dependent for it upon other plants or animals. This food is obtained in two general ways, either (1) directly from the living bodies of plants or animals, or (2) from dead bodies or the products of living bodies. In the first case, in which living bodies are at- tacked, the attacking fungus is called a imrasite^ and the plant or animal attacked is called the liod. In the second case, in which living bodies are not attacked, the fungus is called a saprophyte. Some Fungi can live only as parasites, or as saprophytes, but some can live in either way. Fungi form a very large assemblage of plants, much more numerous than the Alg^. As many of the parasites attack and injure useful plants and animals, producing many of the so-called " diseases," they are forms of great interest. Governments and Experiment Stations have ex- pended a great deal of money in studying the injurious parasitic Fungi, and in trying to discover some method of destroying them or of preventing their attacks. Many of the parasitic forms, however, are harmless ; while many of the saprophytic forms are decidedly beneficial. It is generally supposed that the Fungi are derived from the Algffi, having lost their chlorophyll and power of inde- pendent living. Some of them resemble certain Alga? so closely that the connection seems very plain; but others THALLOrilYTES: FUNGI 49 have been so modified by their parasitic and saprophytic habits that they have lost all likeness to the Algs, and their connection with them is very obscure. 37. The plant body. — Discarding certain problematical forms, to be mentioned later, the bodies of all true Fungi are organized upon a uniform general plan, to which they can all be referred (Fig. 32). A set of colorless branching Pig. 33. A diagrammatic representation of Mvcm\ showing the profusely branching mycelium, and three vertical hyphse (sporophores), sporangia forming on b and c. — After Zopp. filaments, either isolated or interwoven, forms the main working body, and is called the mycelium. The interweav- ing may be very loose, the mycelium looking like a delicate cobweb ; or it may be close and compact, forming a felt-like mass, as may often be seen in connection with preserved fruits. The individual threads are called liy])h(B (singular, liyplia) or liyplial threads. The mycelium is in contact with its source of food supply, which is called the substratum. 50 PLANT STRUCTDEES From the hyphal threads composing the mycelium verti- cal ascending branches arise, which are set apart to produce the asexual spores, which are scattered and produce new mycelia. These branches are called ascending liyplicR or sporoplwres, meaning " spore bearers." Sometimes, especially in the case of parasites, special descending branches are formed, which penetrate the sub- stratum or host and absorb the food material. These spe- cial absorbing branches are called liaustoria, meaning " ab- sorbers." Such a mycelial body, with its sporophores, and perhaps haustoria, lies either upon or within a dead substratum in the case of saprophytes, or vipon or within a living plant or animal in the case of parasites. 38. The subdivisions. — The classification of Fungi is in confusion on account of lack of knowledge. They are so much modified by their peculiar life habits that they have lost or disguised the structures which prove most helpful in classification among the Algae. Four groups will be pre- sented, often made to include all the Fungi, but doubtless they are insuflHcient and more or less unnatural. The constant termination of the group names is mycetes, a Greek word meaning " fungi." The prefix in each case is intended to indicate some important character of the group. The names of the four groups to be presented are as follows : (1) Pliymmycetes (" Alga-Fungi "), referring to the fact that the forms plainly resemble the Algse ; (2) Ascomycetes (" Ascus-Fungi ") ; (3) ^Ecidiomycetes ("^cidium-Fungi ") ; (4) Basidiomycetes (" Basidium-Fungi "). Just what the prefixes ascus, cecidium, and lasidium mean will be ex- plained in connection with the groups. The last three groups are often associated together under the name My- comycetes, meaning " Fungus-Fungi," to distinguish them from the Phycomycetes, or " Alga-Fungi," referring to the fact that they do not resemble the Algae, and are only like themselves. THALLOPHYTES: FUNGI 5]^ One of the ordinary life processes which seems to be seriously interfered with by the saprophytic and parasitic habit is the sexual process. At least, while sex organs and sexual spores are about as evident in Phycomycetes as in Alga3, they are either obscure or wanting in the Mycomycete groups. 1. Phycomycetes {Alga-Fungi) 39. Saprolegnia. — This is a group of " water-moulds," with aquatic liabit like the Alg«. They live upon the dead bodies of water plants and animals (Fig. 33), and some- times attack living fish, one kind being very destructive to young fish in hatcheries. The hyphaj composing the mycelium are coenocytes, as in the Siphon forms. Sporangia are organized at the ends of branches by forming a partition wall separating the cavity of the tip from the general cavity (Pig- 33, B). The tip becomes more or less swollen, and within it are formed numerous biciliate zoospores, which are discharged into the water (Fig. 33, C), swim about for a short time, and rapidly form new mycelia. The process is very suggestive of Cladopliora and Vaucheria. Oogonia and antheridia are also formed at the ends of the branches (Fig. 33, F), much as in Vau- cheria. The oogonia are spherical, and form one and some- times many eggs (Fig. 33, i), E). The antheridia are formed on branches near the oogonia. An antheridium comes in contact with an oogonium, and sends out a deli- cate tube which pierces the oogonium wall (Fig. 33, F). Through this tube the contents of the antheridium pass, fuse with the egg., and a heavy-walled oospore or resting spore is the result. It is an interesting fact that sometimes the contents of an antheridium do not enter an oogonium, or antheridia may not even be formed, and still the egg., without fertiliza- tion, forms an oospore which can germinate. This peculiar 52 PLANT STRUCTURES habit is called imrthenogenesis^ which means reproduction by an Qgg without fertilization. Fig. 33. A common water mould (Saprolegina): A, a fly from which mycelial fila- ments of the parasite aregrowing; B, tip of a branch organized as a sporangium; C sporangium discharging biciliate zoospores; F. oogonium with antheridium in contact, the tube having penetrated to the egg; D and E, oogonia with several eggs.— ^-C after Thuret. D-F nUer DeBary. 40. Mucor. — One of the most common of the Mucors, or " black moulds," forms white furry growths on damp bread, preserved fruits, manure heaps, etc. It is therefore a saprophyte, the coenocytic mycelium branching extensively through the substratum (Fig. 34). THALLOPHYTES: FUNGI 53 Erect sporophores arise from it in abundance, and at the top of each sporophore a globular sporangium is formed, within which are numerous small asexual spores (Figs. 35, Fig. 34. Diagram showing mycelium and sporophores of a common Mucor. — Cald'well. 36). The sporangium wall bursts (Fig. 37), the light spores are scattered by the wind, and, falling upon a suitable sub- stratum, germinate and form new mycelia. It is evident that these asex- ual spores are not zoo- spores, for there is no water medium and swim- ming is impossible. This method of transfer being impossible, the spores are scattered by currents of air, and must be corre- spondingly light and pow- dery. They are usually spoken of simply as " spores," without any prefix. 22 Fig. 35. Forming sporangia of Mucor, show- ing the swollen tip of the sporophore (^4), and a later stage (B), in which a wall is formed separating the sporangium from the rest of the body.— Caldwell. 54 PLANT STRDCTUEES While the ordinary method of reproduction through the growing season is by means of these rapidly germinating spores, in certain conditions a sexual process is observed, by which a heavy-walled sexual spore is formed as a resting spore, able to outlive unfavorable conditions. Branches arise from the hyphge of the mycelium just as in the forma- FiQ. 36. Mature sporangium of Mucor, showing the wall {A), the numerous spores (C), and the columella {B) — that is, the partition wall pushed up into the cavity of the sporangium. —Caldwell. Fig. 37. Bursted sporangium of Mucor, the ruptured wall not being shown, and the loose spores adhering to the colu- mella.—Caldwell. tion of sporophores (Fig. 38). Two contiguous branches come in contact by their tips (Fig. 38, A), the tips are cut oS from the main coenocytic body by partition walls (Fig. 38, i?), the walls in contact disorganize, the contents of the two tip cells fuse, and a heavy-walled sexual spore is the result (Fig. 38, C). It is evident that the process is conjugation, suggesting the Conjugate forms among the THALLOPHYTES: FUNGI 55 Algse ; that the sexual spore is a zygote ; and that the two pairing tip cells cut ofE from the main body by partition walls are gametangia. uMucor, therefore, is isogamous. Fig. 38. Sexual reproduction of Mucor, showing tips of sex branches meeting (A), the two gametangia cut off by partition walls (J5), and the heavy-walled zygote (C)-— Caldwell. 41. Peronospora, — These are the " downy mildews," very common parasites on seed plants as hosts, one of the most common kind attacking grape leaves. The mycelium is coeno- cytic and entirely internal, ramifying among the tissues within the leaf, and piercing the living cells with haustoria which rapidly absorb their contents (Fig. 39). The pres- ence of the parasite is made known by discolored and 56 PLANT STRUCTDRES finally deadened spots on the leaves, where the tissues have been killed. From this internal mycelium numerous sporophores arise, coming to the surface of the host and securing the scattering of their spores, which fall upon other leaves and germinate, the new mycelia pene- trating among the tissues and begin- ning their ravages. The sporophores, af- ter rising above the surface of the leaf, branch freely ; and many of them rising near together, they form little velvety patches on the surface, suggesting the name " downy mildew." b ^ Fig. 39. A branch of Peronoitpora in contact with two cells of a host plant, and sending into them its large haustoria.— After DeBart. Fig. 40. Pei'onospora, one of the Phycomycetes, showing at a an oogonium (o) con- taining an egg, and an antheridinm in) in contact; at b the antheridial tube pene- trating the oogonium and discharging the contents of the antheridium into the egg; at c the oogonium containing the oospore or resting spore.— After DeBart. In certain conditions special branches arise from the mycelium, which organize antheridia and oogonia, and remain within the host (Fig. 40). The oogonium is of the usual spherical form, organizing a single egg. The an- THALLOPHYTES: FUNGI 5Y theridium comes in contact with the oogonium, puts out a tube which pierces the oogonium wall and enters the egg, into which the contents of the antheridium are discharged, and fertilization is effected. The result is a heavy-walled oospore. As the oospores are not for immediate germina- tion, they are not brought to the surface of the host and scattered, as are the asexual spores. When they are ready to germinate, the leaves bearing them have perished and the oospores are liberated. 42. Conclusions. — The coenocytic bodies of the whole group are very suggestive of the Siphon forms among Green Algae, as is also the method of forming oogonia and antheridia. The water-moulds, Saprolegjiia and its allies, have re- tained the aquatic habit of the Alga?, and their asexual spores are zoospores. Such forms as Mucor and Perono- spora, however, have adapted themselves to terrestrial con- ditions, zoospores are abandoned, and light spores are de- veloped which can be carried about by currents of air. In most of them motile gametes are abandoned. Even in the heterogamous forms sperms are not organized within the antheridium, but the contents of the antheridium are discharged through a tube developed by the wall and pene- trating the oogonium. It should be said, however, that a few forms in this group develop sperms, which make them all the more alga-like. They are both isogamous and heterogamous, both zygotes and oospores being resting spores. Taking the characters all together, it seems reasonably clear that the Phycomycetes are an assemblage of forms derived from Green Algae (Chlo- rophyceae) of various kinds. 2. AscoMYCETES {Asci(s- ov Sac-Fungi) 43. Mildews.— These are very common parasites, growing especially upon leaves of seed plants, the mycelium spread- ing over the surface like a cobweb. A very common mil- 58 PLANT STKUCTURES dew, Microsphmra, grows on lilac leaves, which nearly always show the whitish covering after maturity (Fig. 41). The branching hyphae show numerous partition walls, and are not ccenocytic as in the Phycomycetes. Small disk-like haustoria penetrate into the superficial cells of the host, anchoring the mycelium and absorbing the cell contents. Sporophores arise, which form asexual spores in a pe- culiar way. The end of the sporophore rounds off, almost separating itself from the part below, and becomes a spore or spore-like body. Below this another organizes in the same way, then another, until a chain of spores is developed, easily broken apart and scat- tered by the wind. Falling upon other suitable leaves, they germinate and form new mycelia, enabling the fungus to spread rapidly. This meth- od of cutting a branch into sections to form spores is called abstriction, and the spores formed in this way are called conidia^ or conidi- ospores (Fig. 43, B). At certain times the myce- lium develops special branches which develop sex organs, but they are seldom seen and may not always occur. An oogo- nium and an antheridium, of the usual forms, but probably without organizing gametes, come into contact, and as a result an elaborate structure is developed — the ascocarp^ sometimes called the " spore fruit." These ascocarps ap- pear on the lilac leaves as minute dark dots, each one being Fig. 41. Lilac leaf covered with mil- dew {Mic)-ospha:ra), the shaded re- gions representing the mycelium, and the black dots the ascocarps. — S. M. Coulter. TIIALLOrilYTES: FUNGI 59 a little sphere, wliicli suggested the name Microsphmra (Fig. 41). The heavy wall of the ascocarp bears beauti- ful branching hair-like appendages (Fig. 42). Bursting the wall of this spore fruit several very delicate, bladder-like sacs are extruded, and through the transparent wall of each sac there may be seen several spores (Fig. 42). The ascocarp, therefore, is a spore case. Just as is the cystocarp of the Eed Algae (§ 33). The delicate sacs within are the asci, a word meaning "sacs," and each ascus is evidently a mother cell within which asexual spores are formed. These spores are distinguished from other asexual spores by the name ascospore. It is these peculiar moth- er cells, or asci, which give name to the group, and an Ascomycete, Ascus-fungus, or Sac-fungus, is one which pro- duces spores in asci ; and an ascocarp is a spore case which contains asci. In the mildews, therefore, there are two kinds of asexual spores : (1) conidia, formed from a hyphal branch by abstric- tion, by which the mycelium may spread rapidly; and (2) ascospores, formed in a mother cell and protected by a heavy case, so that they may bridge over unfavorable conditions, and may germinate when liberated and form new mycelia. The resting stage is not a zygote or an oospore, as in the Algge and Phycomycetes, no sexual spore probably being formed, but a heavy-walled ascocarp. 44. Other forms. — The mildews have been selected as a simple illustration of Ascomycetes, but the group is a very Fig. 42. Ascocarp of the lilac mUdew, showing branching appendages and two asci protruding from the rup- tured wall and containing ascospores. — S. M. Coulter. 60 PLANT STKCCTUEES large one, and contains a great variety of forms. All of them, however, produce spores in asci, but the asci are not always inclosed by an ascocarp. Here belong the common blue mould {Petiicillimn), found on bread, fruit, etc., in which stage the branching chains of conidia are very con- spicuous (Fig. 43) ; the truffle-fungi, upon whose subter- Fie. 43. PenicilUum, a common mould: A, mycelium with numerous branching sporophores bearing conidia; B, apex of a sporophore enlarged, showing branch- ing and chains of conidia. — After Brbfeld. ranean mycelia ascocarps develop which are known as " truffles " ; the black fungi, which form the diseases known as " black knot " of the plum and cherry, the " ergot " of rye (Fig. 44), and many black wart-like growths upon the bark of trees ; other forms causing " witches'-brooms " (ab- normal growths on various trees), " peach curl," etc., the cup-fungi (Figs. 45, 46), and the edible morels (Fig. 47). THALLOPIIYTES: FUNGI 61 Fis. 44. Head of rye attacked by "er- got" (a), peculiar grain-like masses replacing the grains of rye ; also a mass of "ergot" germinating to form spores (4).— After Tclasne. Fig. 46. A cup-fungus (Pitya) grow- ing on a spruce {Picea). — After Rehm. In some of these forms the ascocarp is completely closed, as in the lilac mildew ; in others it is flask-shaped ; in others, as in the cup-fungi, it is like a cup or disk ; hut in all the spores are inclosed by a delicate sac, the ascus. 62 PLANT STRUCTURES Here must probably be included the yeast-fungi (Fig. 48), so commonly used to excite alcoholic fermentation. Fig. 47. The common edible morel (Morchella esctilenta). The structure shown and used represents the ascocarp, the depressions of whose surface are lined with asci contain- ing ascospores. — After Gibson. Fig. 48. Yeast cells, reprodu- cing by budding, and form- ing chains.— Caldwell. The " yeast cells " seem to be conidia having a peculiar bud- ding method of multiplication, and the remarkable power of exciting alcoholic fermentation in sugary solutions. 3. ^ciDiOMYCETES {^cidium-Fungi) 45. General characters. — This is a large group of very destructive parasites known as " rusts " and " smuts." The rusts attack particularly the leaves of higher plants, pro- ducing rusty spots, the wheat rust probably being the best known. The smuts especially attack the grasses, and are very injurious to cereals, producing in the heads of oats, barley, wheat, corn, etc., the disease called smut. TIIALLOPIIYTES: FUNGI g3 In some forms an obscure sexual process has been de- scribed, but it is beyond the reach of ordinary observation. The iEcidiomycetes do not form an independent and nat- ural group, but are now generally placed under the Basi- diomycetes, but they are so unlike the ordinary forms of that group that they are here kept distinct. Most of the forms are Yerj poly7)iorpInc — that is, a plant assumes several dissimilar appearances in the course of its life history. These phases are often so dissimilar that they have been described as different plants. This polymorphism is often further complicated by the appearance of different phases upon entirely different hosts. For example, the wheat-rust fungus in' one stage lives on wheat, and in an- other on barberry. 46. Wheat rust. — This is one of the few rusts whose life histories have been traced, and it may be taken as an illus- tration of the group. The mycelium of the fungus is found ramifying among the leaf and stem tissues of the wheat. While the wheat is growing this mycelium sends to the surface numerous spo- c^^^fkrm^'^(?Sfi iliiL" Fig. 49. Wheat rust, showing sporophores breaking through the tissues of the host and bearing summer spores (uredospores). — After H. Marshall Ward. rophores, each bearing at its apex a reddish spore (Fig. 49). As the spores occur in great numbers they form the rusty- looking lines and spots which give name to the disease. The spores are scattered by currents of air, and falling upon other plants, germinate very promptly, thus spreading the 64 PLANT STRUCTUEES disease with great rapidity (Fig. 50). Once it was thought that this completed the life cycle, and the fungus received the name Uredo. When it was known that this is but one Fig. 50. — Wheat rust, showing a young hypha forcing its way from the surface of leaf down among the nutritive cells. — After H. Marshall Wabd. stage in a polymorphic life history it was called the Uredo- stage, and the spores iiredospores, sometimes " summer spores.^' Fig. 51. Wheat rust, showing the winter spores (teleutospores).— After H. Marshall Ward. Toward the end of the summer the same mycelium develops sporophores which bear an entirely different kind of spore (Fig. 51). It is two-celled, with a very heavy black THALLOPIIYTES: FUNGI 65 wall, and forms what is called the " black rust," which ap- pears late in the summer on wheat stubble. Tliese spores are the resting spores, which last through the winter and germinate in the following spring. They are called teUuto- spores, meaning the " last spores " of the growing season. They are also called " winter spores," to distinguish them from the uredospores or " summer spores." At first this teleutospore-bearing mycelium was not recognized to be identical with the uredospore-bearing mycelium, and it was called Puccinia. This name is now retained for the whole polymorphous plant, and wheat rust is Puccinia granmiis. This mycelium on the wheat, with its summer spores and winter spores, is but one stage in the life history of wheat rust. In the spring the teleutospore germinates, each cell developing a small few-celled filament (Fig. 53). From each cell of the filament a little branch arises which develops at its tip a small spore, called a sjjo- ridium, which means " spore-like." This little filament, which is not a parasite, and which bears sporidia, is a second phase of the wheat rust, really the first phase of the growing season. The sporidia are scattered, fall upon barberry leaves, germinate, and develop a mycelium which spreads through the leaf. This mycelium produces sporophores which emerge on the under surface of the leaf in the form of chains of reddish-yellow conidia (Fig. 53). These chains of conidia are closely packed in cup-like receptacles, and these reddish-yellow cup-like masses are often called Fig. 52. Wheat rtist, show- ing a teleutospore germina- ting and forming a short fil- ament, from four of whose cells a spore branch arises, the lowest one bearing at its tip a sporidinm. — After H. Marshall Ward. QQ PLANT STRUCTURES " cluster-cups." This mycelium on the barberry, bearing cluster-cups, was thought to be a distinct plant, and was called jl^Jcidium. The name now is applied to the cluster-cups, which are called cecidia, and the conidia-like spores which they produce are known as (ecidiospores. It is the gecidia which give name to the group, and ^cidiomycetes are those Fungi in whose life history gecidia or cluster-cups appear. The ajcidiospores are scattered by the wind, fall upon the spring wheat, germinate, and develop again the myce- lium which produces the rust on the wheat, and so the life cycle is com- pleted. There are thus at least three distinct stages in the life history of wheat rust. Begin- ning with the growing season they are as fol- lows : (1) The phase bear- ing the sporidia, which is not parasitic ; (2) the eecidium phase, parasitic on the barberry; (3) the uredo-teleutospore phase, para- sitic on the wheat. In this life cycle at least four kinds of asexual spores TIIALLOPIIYTES: FUNGI 67 appear : (1) sporidia, which develop the stage on the barber- ry ; (3) (ecidiospores, which develop the stage on the wheat ; (3) uredos2iores^yfhich. repeat the mycelium on the wheat ; (4) teleutos2)ores,yf\\\ch last through the winter, and in the spring produce the stage bearing sporidia. It should be said that there are other structures of this plant produced on the bar- berry (Fig. 53), but they are too uncertain to be included here. The barberry is not absolutely necessary to this life cycle. In many cases there is no available barberry to act as host, and the sporidia germinate'directly upon the young wheat, forming the rust-producing mycelium, and the cluster-cup stage is omitted. Fig. 54. Two species of "cedar apple" {Gymnosporanglum). both on the common juniper (Juniperns Virginiana).—A after Farlow, B after Engler and Prantl. 47. Other rusts. — Many rusts have life histories similar to that of the wheat rust, in others one or more of the stages are omitted. In very few have the stages been con- 68 PLANT STRUCTDKES nected together, so that a mycelium bearing uredospores is called a Uredo, one bearing teleutospores a Puceinia, and one bearing aecidia an ^cidiuin ; but what forms of Uredo, Puccinia^ and JEcidium belong together in the same life cycle is very difficult to discover. Another life cycle which has been discovered is in con- nection with the " cedar apples " which appear on red cedar (Fig. 54). In the spring these diseased growths be- come conspicuous, especially after a rain, when the jelly- like masses containing the orange-colored spores swell. This corresponds to the phase which produces rust in wheat. On the leaves of apple trees, wild crab, hawthorn, etc., the aecidium stage of the same parasite develops. 4. Basidiomycetes {Basidium-Fungi). 48. General characters. — This group includes the mush- rooms, toadstools, and puffballs. They are not destructive parasites, as are many forms in the preceding groups, but mostly harm- less and often useful sap- rophytes. They must also be regarded as the most highly organized of the Fungi. The popular distinction between toad- stools and mushrooms is not borne out by botan- ical characters, toadstool and mushroom being the same thing botanically, and forming one group, puffballs forming an- other. As in ^cidiomycetes, Fig. 55. The common edible mushroom, Agaricus campestris.—Afier Gibson. an obsCUre Sexual prOCeSS TIIALLOPIIYTES: FUNGI 69 is reported. The life history seems simple, but this appar- ent simplicity may represent a very complicated history. The structure of the common mushroom {Agaricus) will serve as an illustration of the group (Fig. 55). 49. A common mushroom. — The mycelium, of white branching threads, spreads extensively through the decay- ing substratum, and in cultivated forms is spoken of as the " spawn." Upon this myce- lium little knob- like protuberances begin to arise, grow- ing larger and larger, until they are organized into the so-called " mushrooms." The real body of the plant is the white thread - like mycelium, while the " mushroom " part seems to rep- resent a great num- ber of sporophores organized together to form a single complex spore- bearing structure. The mushroom 23. Fig. 56. A common Agariciis : A, section through one side of pik'us, showing sections of the pendent gills; B. section of a gill more enlarged, showing the cen- tral tissue, and the broad border formed by the ba- sidia : C, still more enlarged section of one side of a gill, showing the club-shaped basidia standing at right angles to the surface, and sending out a pair of small branches, each of which bears a single ba- sidiospore.— After Sachs. w% jWl ^HVv ^ ^ ^Bk: ^1 HH|| ^ ^K^^l ^^^^1 'il Ifc'^ ^^m..-:M- -■■ jsM ^^^^^^^^^1 ii\ V^\ s.^^1 ^Hf|H^^4^^ ^^^^^^B i ;1 ^v^|H ^^K -; '''■' ^-^3^H m^^i ii i^^l c o cc <>7 "3 5^ THALLOPIIYTES: FUNGI 71 has a stalk-like portion, the stij^e, at the base of which the slender mycelial threads look like white rootlets ; and an expanded, umbrella-like top called the pileus. From the under surface of the pileus there hang thin radiating plates, or gills (Fig. 55). Each gill is a mass of interwoven fila- ments (hyphae), whose tips turn toward the surface and form a compact layer of end cells (Fig. 56). These end Fig. 60. A bracket fungus (Polyporus) growing on the trunk of a red oak.- Caldwell. cells, forming the surface of the gill, are club-shaped, and are called basidin. From the broad end of each basidium two or four delicate branches arise, each bearing a minute spore, very much as the sporidia appear in the wheat rust. 12 PLANT STRUCTUEES These spores, called basidiospores, shower down from the gills when ripe, germinate, and produce new mycelia. The peculiar cell called the basidium gives name to the group Basidiomycetes. 50. Other forms. — Mushrooms display a great variety of form and coloration, many of them being very attractive Fig. 61. A toadstool of the bracket form which has grown about blades of grass without interfering with their activity.— Caldwell. (Figs. 57, 58, 59). The " pore-fungi " have pore-like depres- sions for their spores, instead of gills, as in the very com- mon "bracket-fungus" {Polyporus), which forms hard shell-like outgrowths on tree-trunks and stumps (Figs. 60, Fig. 62. The common edible Boletus (B.edu- Fig. 63. Another edible Boletus {B. .ilro- lis), in which the gills are replaced by bilaceiis).— After Gibson. pores.— After Gibson. Fio— S— 0— CT=:g>o— S— 0— G, etc. The formula indicates that the gametophyte produces two gametes (sperm and Qgg), which fuse to form an oospore, which produces the sporophyte, which produces an asexual spore, Avhich produces a gametophyte, etc. That alternation of generations is of great advantage is evidenced by the fact that it appears in all higher plants. It must not be supposed that it appears first in the Bryo- phytes, for its beginnings may be seen among the Thallo- phytes. The Bryophytes, however, first display it fully organized and without exception. Just what this alterna- tion does for plants may not be fully known, but one advantage seems prominent. By means of it many gameto- phytes may result from a single oospore • in other words. 98 PLANT STRDCTUKES it multiplies the product of the sexual spore. A glance at the formula given above shows that if there were no sporo- phyte (S) the oospore would produce but one gametophyte (G). By introducing the sporophyte, however, as many gametophytes may result from a single oospore as there are asexual spores produced by the sporophyte, which usually produces a very great number. In reference to the sporophytes and gametophytes of Bryophytes two peculiarities may be mentioned at this point : (1) the sporophyte is dependent upon the gameto- phyte for its nourishment, and remains attached to it ; (2) the gametophyte is the special chlorophyll -generation, and hence is the more conspicuous. It follows that, in a general way, the sporophyte of the Bryophytes only pro- duces spores, while the gametophyte both produces gametes and does chlorophyll work. It is important also to note that the protected resting stage in the life history is not tlie sexual spore, as in the Algse, but is the asexual spore in connection with the sporophyte. These spores have a protecting wall, are scattered, and may remain for some time without germi- nation. If the ordinary terms in reference to Mosses be fitted to the facts given above, it is evident that the "moss plant " is the leafy branch of the gametophyte ; that the ''moss fruit" is the sporophyte; and that the alga- like part of the gametophyte has escaped attention and a popular name. The names now given to the different structures which appear in this life history are as follows : The alga-like part of the gametophyte is the protonema, the leafy branch is the gametopJiore (''gamete-bearer ") ; the whole sporophyte is the sjyprogonium (a name given to this peculiar leafless sporophyte of Bryophytes), the stalk-like portion is the seta, the part of it imbedded in the gametophore is the foot, and the urn-like spore-case is the capsule. BKYOPHYTES 99 63. The antheridium.— The male organ of the Bryophjtes is called an antheridium, just as among Thallophytes, but it has a very different structure. In general among the ^lliib. Fig. 83. Sex organs of a common moss (Funaria): the group to the right represents an antheridium (A) discharging from its apex a mass of sperm mother cells (a), a single mother cell with its sperm (6), and a single sperm (c). showing body and two cilia; the group to the left represents an archegonial cluster at summit of stem (.4).' showing archegonia (a), and paraphyses and leaf sections (b), and also a single archegoninm (£). with venter (b) containing egg and ventral canal cell, and neck (h) containing the disorganizing axial row (neck canal cells').— After Sachs. Thallophytes it is a single cell (mother cell), and may be called a simple antheridium, but in the Bryophytes it is a many-celled organ, and may be regarded as a compound antheridium. It is usually a stalked, club-shaped, or oval to 100 PLANT STRUCTURES globular body (Figs. 83, 84, 103). A section through this body shows it to consist of a single layer of cells, which forms the wall of the antheridium, and within this a com- pact mass of small cubical (square in section) cells, within each one of which there is formed a single sperm (Fig. 84). These cubical cells are evidently moth- er cells, and to distinguish them from others they are called sperm, mother cells. An antheridium, therefore, aside from its stalk, is a mass of sperm mother cells surrounded by a wall consisting of one layer of cells. The sperm is a very small cell with two long cilia (Fig. 83). The two parts are spoken of as "body" and cilia, and the body may be straight or somewhat curved. These small bicili- ate sperms are one of the distinguish- ing marks of the Bryophytes. The existence of male gametes in the form of ciliated sperms indicates that fertil- ization can take place only in the pres- ence of water, so that while the plant has become terrestrial, and its asexual spores have respond- ed to the new conditions and are no longer ciliated, its sexual process is conducted as among the Green Algae. It must not be supposed, however, that any great amount of water is necessary to enable sperms to swim, even a film of dew often answering the purpose. When the mature antheridia are wet they are opened at the apex and discharge the mother cells in a mass (Figs. 83, 105, E), the walls of the mother cells become mucilagi- nous, and the sperms escaping swim actively about and are attracted to the organ containing the egg. 64. The archegonium. — This name is given to the female sex organ, and it is very different from the oogonium of Fig. 84. Antheridium of a liverwort in section, showing single layer of wall cells surround- ing the mass of moth- er cells. — After Stras- BURGER. BRYOPIIYTES 101 Thallophytes. Instead of being a single mother cell, it is a many-celled structure, shaped like a flask (Figs. 83, 98). The neck of the flask is more or less elongated, and within the bulbous base (venter) the single egg is organized. The archegonium, made up of neck and venter, consists mostly of a single layer of cells. This hollow flask is solid at first, there being a central vertical row of cells surrounded by the single layer just referred to. All of the cells of this axial row, except the lowest one, disorganize and leave a passageway down through the neck. The lowest one of the row, which lies in the venter of the archegonium, or- ganizes the egg. In this way there is formed in the arche- gonium an open passageway through the neck to the egg lying in the venter. To this neck the swimming sperms are attracted, enter and pass down it, one of them fuses with the egg, and this act of fertilization results in an oospore. Archegonia and antheridia are supposed to have been derived from a many-celled gametangium, such as occurs in certain Brown Algae (Fig. 18). The presence of the archegonia is one strong and unvarying distinction between Thallophytes and Bryophytes. Pteridophytes also have archegonia, and so characteristic an organ is it that Bryo- phytes and Pteridophytes are spoken of together as Arclie- goniatc)^. 65. Germination of the oospore. — The oospore in Bryo- phytes is not a resting spore, but germinates immediately by cell division, forming the sporophyte embryo, which presently develops into the mature sporophyte (Fig. 85,^). The lower part of the embryo develops the foot, which ob- tains a firm anchorage in the gametophore by the latter growing up around it (Fig. 85, B, C). The upper part of the embryo develops the seta and capsule. As the embryo increases in size, the venter of the archegonium grows also, forming what is called the C(dy2)tra ; and in true Mosses the embryo presently breaks loose the calyptra at its base 25 102 PLANT STRUCTURES and carries it upward perched on the top of the capsule like a loose cap or hood (Figs. 82, c, 107), which sooner or later falls ofE. As stated be- fore, the mature struc- ture developed from the oospore is called a sporogouium, a form of sporophyte peculiar to the Bryophytes. 66. The sporogonimn. — In its fullest devel- opment the sporogoui- um is differentiated into the three regions, foot, seta, and capsule (Figs. 82, 107) ; but in some forms the seta may be lacking, and in others the foot also, the sporogouium in this last case being only the capsule or spore case, which, after all, is the essential part of any sporogouium. At first the capsule is solid, and its cells are all alike. Later a group of cells within begins to differ in ap- pearance from those about them, being set apart for the produc- tion of spores. This initial group of spore-producing cells is called the arche- spormm, a word meaning "the beginning of spores." It Fig. 85. Sporogouium of Fimaria : A, an em- bryo sporogonium if,/'), developiug within the venter (6, 6) of an archegonium ; B, C, tips of leafy shoots bearing young sporo- gonia, pushing up calyptra (c) and archego- nium neck {h), and sending the foot down Into the apex of the gametophore.— After GOEBEL. BRYOPIIYTES 103 does not follow that the archesporial cells themselves pro- duce spores, but that the spores are to appear sooner or later in their progeny. Usually the archesporial cells divide and form a larger mass of spore-producing cells. Such cells are known as s2Jorogenous ("spore-producing") cells, or the group is spoken of as sporogenous tissue. Spo- rogenous cells may divide more or less, and the cells of the last division are mother cells, those which directly produce the spores. The usual sequence, therefore, is archesporial cells (archesporium), sporogenous cells, and mother cells ; but it must be remembered that they all may be referred to as sporogenous cells. Each mother cell organizes within itself four spores, the group being known as a tetrad. In Bryophytes and the higher groups asexual spores are always produced in tetrads. After the spores are formed the walls of the mother cells disorganize, and the spores are left lying loose in a cavity which was formerly occupied by the sporoge- nous tissue. All mother cells do not always organize spores. In some cases some of them are used up in supplying nour- ishment to those which form spores. Such mother cells are said to function as nutritive cells. In other cases, certain mother cells become much modified in form, being organ- ized into elongated, spirally-banded cells called ektters (Figs. 97, 101), meaning "drivers" or "hurlers." These elaters lie among the loose ripe spores, are discharged with them, and by their jerking movements assist in scattering them. The cells of the sporogonium which do not enter into the formation of the archesporium, and are not sporoge- nous, are said to be sterile, and are often spoken of as sterile tissue. Every sporogonium, therefore, is made up of sporogenous tissue and sterile tissue, and the differences found among the sporogonia of Bryophytes depend upon the relative display of these two tissues. The sporogonium is a very important structure from the standpoint of evolution, lor it represents the conspicu- 104 PLANT STRUCTURES ous part of the higher plants. The " fern plant," and the herbs, shrubs, and trees among "flowering plants" correspond to the sporogonium of Bryophytes, and not to the leafy branch (gametophore) or "moss plant." Conse- quently the evolution of the sporogonium through the Bryophytes is traced with a great deal of interest. It may be outlined as follows : In a liverwort called Kiccia the simplest sporogonium is found. It is a globular capsule, without seta or foot Fig. 86. Diagrammatic sections of sporogoiiia of liverworts: A. Riccia, the whole capsule being archesporium except the sterile wall layer ; B, Marchantia, one half the capsule being sterile, the archesporium restricted to the other half ; Z>, Anthoceros, archesporium still more restricted, being dome-shaped and capping a central sterile tissue, the columella (col). — After Gobbel. (Fig. 86, A). The only sterile tissue is the single layer of cells forming the wall, all the cells within the wall be- longing to the archesporium. The ripe sporogonium, therefore, is nothing but a thin-walled spore case. It is well to note that the sporophyte thus begins as a spore case, and that any additional structures that it may de- velop later are secondary. In another liverwort (M(irclianfia) the entire lower half of the sporogonium is sterile, while in the upper half there BRYOi'IIYTES 105 is a single layer of sterile cells as a wall about the arche- sporium, which is composed of all the remaining cells of the upper half (Fig. 8G, B). It will be noted that the sterile tissue in this sporogonium has encroached upon the arche- sporium, which is restricted to one half of the body. In this case the archesporium has the form of a hemisphere. In another liverwort {Jungermamria) the archesporium is still more restricted (Fig. 87). The sterile tissue is organ- FiG. 87. Diagrammatic section of i?po- rogoninm of a Jungermannia form, showing differentiation into foot, seta, and capsule, the archesporium restricted to upper part of sporogo- nium.—After GOEBEL. Fig. 88. Section through sporogonium of Sphagnum, showing capsule (k) with old archegonium neck (ah), calyptra (ca), dome-shaped mass of sporogenous tissue (spo). and columella {co). also the bulb- ous foot is])/) imbedded in the pseudo- podium (;)*■).— After ScuiMrEit. ized into a foot and a seta, and the archesporium is a com- paratively small mass of cells in the upper part of the sporogonium. In another liverwort {Aui//(iceros) the sterile tissue or- ganizes foot and seta, and the archesporium is still more restricted (Fig. 8G, D). Instead of a solid hemispherical 106 PLANT STEUCTUKES % mass, it is a dome-shaped mass, the inner cells of the hemi- sphere having become sterile. This central group of sterile cells which is surrounded by the ar- chesporium is called the columella, which means "^a small column." In a moss called Sphagnum there is the same dome-shaped archespori- um with the columella, as in Ati- thoceros, but it is relatively smaller on account of the more abundant sterile tissue (Fig. 88). In the highest Mosses the arche- sporium becomes very small as com- pared with the sterile tissue (Fig. 89). A foot, a long seta, and an elaborate capsule are organized from the sterile tissue, while the arche- sporium is shai^ed like the walls of a barrel, as though the dome-shaped archesporium of Sphagnum or An- thoceros had become sterile at the apex. In this way the columella is continued through the capsule, and is not capped by the archesporium. This series indicates that after the sporogonium begins as a simple spore case {Riccia), its tendency is to increase sterile tissue and to re- strict sporogenous tissue, using the sterile tissue in the formation of the organs of the sporogonium body, as foot, seta, capsule walls, etc. Among the Green Algfe there is a form known as Coleochcete, whose body resembles those of the sim- plest Liverworts (Fig. 90). When Fig. 89. Young sporofroni- um of a true moss, show- ing foot, seta, and young capsule, in which the ar- chesporium (darker por- tion') is barrel-shaped, and through it the columella is continuous with the lid. — After Campbell. BRYOPIIYTES 107 its oospores germiiifite there is formed a globular mass of cells, every one of which is a spore mother cell (Fig. 90, C). If an outer layer of mother cells should become sterile and form a wall about the others, such a spore case as that of Fig. QO.—Coleoch(Efe, one of the green algae: A, a portion of the thallus, showing oogonia with trichogynes (og), antheridia {««), and two enlarged biciliate sperms (3); B, a fertilized oogonium containing oospore and invested by a tissue (r) which has developed after fertilization ; C, an oospore which has germinated and formed a mass of cells (probably a sporophyte), each one of which organizes a biciliate zoospore (Z)).— After Pringsheim. Riccia would be the result (Fig. 86, A). For such reasons many believe that the Liverworts have been derived from such forms as CoIeocJia'te. 67. The gametophyte. — Having considered the sporo- phyte body as represented by the sporogonium, we must consider the gametophyte body as represented by proto- nema and leafy branch (gametophore). The gametophyte results from the germination of an asexual spore, and in the Mosses it is differentiated into protonema and leafy gametophore (Figs. 81, 83, 102). Like the sporophyte, IQg PLANT STKUCTUKES however, it shows an interesting evolution from its sim- plest condition in the Liverworts to its most complex con- dition in the true Mosses. In the Liverworts the sj)ore develops a flat thallus body, one plate of cells or more in thickness, which generally branches dichotomously (see § 29) and forms a more or less extensive body (Fig. 92). This thallus is the gametophyte, there being no differentiation into protonema and leafy branch. In the simpler Liverworts the sex organs (antheridia and archegonia) are scattered over the back of this thallus (Fig. 92). In other forms they become collected in certain definite regions of the thallus. In other forms these defi- nite sexual regions become differentiated from the rest of the thallus as disks. In other forms these disks, bearing the sex organs, become short-stalked, and in others long- stalked, until a regular branch arises from the thallus body (Figs. 96, 97). This erect branch, bearing the sex or- gans, is, of course, a gametophore, but it is leafless, the thallus body doing the chlorophyll work. In the Sphagnum Mosses the spore develops the same kind of flat thallus (Fig. 104), but the gametophore be- comes leafy, sharing the chlorophyll work with the thallus. In the true Mosses most of the chlorophyll work is done by the leafy gametophore, and the flat thallus is reduced to branching filaments (the protonema) (Fig. 102). The protonema of the true Mosses, therefore, corre- sponds to the flat thallus of the Liverworts and SpJiagtinm, while the leafy branch corresponds to the leafless gameto- phore found in some Liverworts. It also seems evident that the gametophore was originally set apart to bear sex organs, and that the leaves whicli appear upon it in the Mosses are subsequent structures. CHAPTEE VIII THE GREAT GROUPS OF BRYOPHYTES Hepatic^ (Liverworts) 68. General character. — Liverworts live in a variety of conditions, some floating on the water, many in damp places, and many on the bark of trees. In general they are moistnre-loving plants (hydrophytes), though some can en- dure great dryness. The gametophyte body is prostrate, though there may be erect and leafless gametophores. This prostrate habit develops a dorsiventral body — that is, one whose two surfaces {dorsal and vetitral) are exposed to different conditions and become unlike in structure. In Liverworts the ventral surface is against the substratum, and puts out hair-like processes {rhizoids) for anchorage and possibly absorption. The dorsal region is exposed to the light and its cells develop chlorophyll. If the thalhis is thin, chlorophyll is developed in all the cells ; if it be so thick that the light is cut off from the ventral cells, the thallus is differentiated into a green dorsal region doing the chlorophyll work, and a colorless ventral region producing anchoring rhizoids. This latter represents a simple differ- entiation of the nutritive body into working regions, the ventral region absorbing material and conducting it to the green dorsal cells which use it in making food. There seem to have been at least three main lines of development among Liverworts, each beginning in forms with a very simple thallus, and developing in different di- rections. They are briefly indicated as follows : 109 110 PLANT STRUCTURES G9. Marchantia forms. — lu this line the simple thallus gradually becomes changed into a very complex one. The thallus retains its simple outlines, but becomes thick and differentiated in tissues (groups of similar cells). The line may be distin- guished, therefore, as one in which the differentia- tion of the tissues of the gametophyte is emphasized (Figs. 91-93). In Mar- cliantia proper the thallus becomes very complex, and it may be taken as an illus- tration. The thallus is so thick that there are very distinct green dorsal and colorless ventral regions (Fig. 94). The latter puts out numerous rhizoids and scales from the single layer of epidermal cells. Above the ventral epidermis are several layers of colorless Fig. 91. A very small species of Riccia, one of the Marchantia forms : A, a group of thallus bodies slightly en- larged ; B, section of a thallus, show- ing rhizoids and two sporogonia im- bedded and communicating with the outside by tubular passages in the thallus. — After Strasburger. Fig. 92. Ricciocarpus, a Marchantia form, 8ho\Ving numerons rhizoids from ventral surface, the dichotomous branching, and the position of the sporogonia on the dorsal surface along the "midribs."— Goldbekger. Fig. 93. Two common liverworts : to the left is Conocephalus, a Marchantia form, showing rhizoids, dichotomous branching, and the conspicuous rhombic areas (areolae) on the dorsal surface; to the right is Anthoceros, with its simple thallus and pod-like sporogonia.— Goldberger. Ficj. 94. Cross-sections of thallus of Marchantia: A, section from thicker part of thallus, where supporting ti-ssiic {p] is abundant, and showing lower epidermis giving rise to rhizoids (/;) and plates (J), also chlorojihyll tissue (chl) organized into chambers by partitions (oi; B, section near margin of thallus more magnified, eliowing lower epidermis, two layers of supi)orting tissue ip) with reticulate walls, a single chloroi)hyll chamber with its bounding walls is) and containing short, often branching filaments whose cells contain chloroplasts {chl), overarching upper epidermis (o) pierced by a large chimney-like air-pore («/>).— After Goebel. Fig. 95. Section through cupule of ilarchantia, showing wall in which are chloro- phyll-bearing air-chambers with air-pores, and gemmie (a) in various stages of development. — Dodei.-Port. Fig. 96. Marchantia polymorpha : the lower figure represents a gametophyte bear- ing a mature antheridial branch (d), some ybung antheridial branches, and also Bome cupules with toothed margins, in which the gemmae may be seen ; the upper figure represents a partial section through the antheridial disk, and shows antheridia within the antheridial cavities (a, b, c, d, €,/).— After Knt. THE GREAT GROUPS OF BRYOPIIYTES 113 cells more or less modified for conduction. Above these the dorsal region is organized into a series of large air cham- bers, into which project chlorophyll-containing cells in the Fig. 97. Marchantia polymorjiha, a common liverwort : i, thallus. witli rhizoids, bearing a mature archegonial branch (/) and several younger ones (a, h, c, d, e); S and 3, dorsal and ventral views of archegonial disk; U and 5. young sporophyte (sporogonium) embryos; G. more mature sporogonium still within enlarged venter of archcgonium; 7, mature sporogonium discharging spores; S, three spores and an ulatcr. — After Kny. form of short branching filaments. Overarching the air chambers is the dorsal epidermis, and piercing through it into each air chamber is a conspicuous air pore (Fig. 94, B). 114 PLANT STRUCTUKES The air chambers are outlined on the surface as small rhombic areas (areolw), each containing a single air pore. Peculiar reproductive bodies are also developed upon the dorsal surface of Marcltantia for vegetative multiplica- FiG. 98. Marchantia poiytnori-)ha: i, partial section thronph archegonial disk, ehow- ing arcliegonia with long necks, and venters containing eggs; ,9, young archego- niiim showing axial row; 10, superficial view at later stage; 11, mature archego- nium, with axial row disorganized and leaving an open passage to the large egg; n, cross-section of venter; 13, cross-section of neck.— After Knt. tion. Little cups (cnpules) appear, and in them are numer- ous short-stalked bodies {gemnm), which are round and flat (biscuit-shaped) and many-celled (Figs. 95, 96). The THE GREAT GROUPS OF BRYOPIIYTES H^ gemmae fall oif and develop new thallus bodies, making rapid multiplication possible. Marchantia also possess remarkably prominent gameto- pliores, or "sexual branches" as they are often called. In this case the gametophores are differentiated, one bear- ing only antheridia (Fig. 96), and known as the "anthe- ridial branch," the other bearing only archegonia (Figs. 97, 98), and known as the ''archegonial branch." The scal- loped antheridial disk and the star-shaped archegonial disk, each borne up by the stalk-like gametophore, are seen in the illustrations. Not only are the gametophores sexually dif- ferentiated, but as only one appears on each thallus, the thal- lus bodies are sexually differentiated. When the two sex organs appear upon different individuals, the j)lant is said to be dimcious, meaning " two households " ; when they both appear upon the same individual, the plant is monoecious, meaning " one household." Some of the Bryophytes are mo- noecious, and some of them are dioecious (as 3Iarchantia). Another distinguishing mark of the line of Marchantia forms is that the capsule-like sporogonium opens irregu- larly to discharge its spores (Fig. 97, 7). 70. Jungermannia forms. — This is the greatest line of the Liverworts, the forms being much more numerous than in the other lines. They grow in damp places ; or in drier situations on rocks, ground, or tree-trunks ; or in the tropics also on the leaves of forest plants. They are gen- erally delicate plants, and resemble small Mosses, many of them doubtless being commonly mistaken for Mosses. This resemblance to Mosses suggests one of the chief features of the line. Beginning with a simple thallus, as in the Marcliantia line, the structure of the thallus re- mains simple, there being no such differentiation of tissues as in the May'chayitia line ; but the form of the thallus becomes much modified (Figs. 99, 100). Instead of a flat thallus with even outline, the body is organized into a cen- tral stem-like axis bearing two rows of small, often crowded 116 PLANT STRUCTURES leaves. There are really three rows of leaves, but the third is on the ventral side against the substratum, and is often so much modified as not to look like the other leaves. In consequence of this the Jungermannia forms are usually called "leafy liverworts," to distinguish them from the Fig. 99. Two liverworts, both Jungermannia forms: to the left is Blasia, which re- tains the thallus form but has lobed margins; to the right is Scapa?na, with dis- tinct leaves and sporogonia (-4).— Goldberger. other Liverworts, which are '^thallose." They are also often called "scale mosses," on account of their moss-like appearance and their small scale-like leaves. The line may be distinguished, therefore, as one in which the differentiation of the form of the gametophyte is emphasized. Another distinguishing mark is that the sporogonium has a prominent seta, and the capsule splits down into four pieces (valves) when opening to discharge the spores (Fig. 100, C). 71. Anthoceros forms. — This line contains comparatively few forms, but they are of great interest, as they are sup- posed to represent forms which have given rise to the THE GKEAT GKOUPS OF BKYOFHYTES H'j Fig. 100. Species of Lepidosia, a genus of leafy liverworts, showing different leaf forms, and in A and C the dehiscence of the sporogoniiim by four valves In C rhizoids are evident; and in B, D, and E the three rows of leaves are seen, the leaves of the ventral row being comparatively small.-After Engler and Prastl. -Mosses, and possibly to the Pteridophytes also. The thallus is very simple, being differentiated neither in structure nor form, as in the two other lines ; but the 26 118 TLANT STRUCTURES special development has been in connection with the sporogonium (Figs. 93, 101). This complex sporogonium (sporophyte) has a large bulbous foot imbedded in the simple thallus, while above there arises a long pod-like capsule. The com- plex walls of this cap- sule contain chlorophyll and air pores, so that the sporogonium is or- ganized for chlorophyll work. If it could send absorbing roots into the soil, this sporophyte could live independent of the gametophyte. In opening to discharge spores the pod-like cap- sule splits down into two valves. Another peculiarity of the Anthoceros forms is in connection with i ji W ' ) ' M ^ M ^^^® antheridia and arch- \v 11 m-^.~) ^^^,^ ^^&^ M egonia. These organs, instead of growing out free from the body of the thallus, as in other Liv- erworts, are imbedded in it. The significance of this peculiarity lies in the fact that it is a char- acter which belongs to the Pteridophytes. The chief direction of development of the three liv- erwort lines may be summed up briefly as follows : The MarcJiantia line has differentiated the structure of the Fig. 101. Anthoceros gracilis : A, several gametophytes, on which sporogonia have developed ; J5, an enlarged sporogonium, showing its elongated character and de- hiscence by two valves leaving exposed the slender columella on the surface o^ which are the spores; C, D, E, F, ela- ters of various forms ; G, spores. — After ScniFFNER. THE GEEAT GROUPS OF 'BEYOPHYTES HQ gametophyte ; the Jungermannia line has differentiated the form of the gametophyte ; the Anthoceros line has differentiated the structure of the sporophyte. It should be remembered that other characters also serve to distin- guish the lines from one another. Musci (Mosses) 72. General character. — Mosses are highly specialized plants, probably derived from Liverworts, the numerous forms being adapted to all conditions, from submerged to very dry, being most abundantly displayed in temperate and arctic regions. Many of them may be dried out com- pletely and then revived in the presence of moisture, as is true of many Lichens and Liverworts, with which forms Mosses are very commonly associated. They also have great power of vegetative multiplica- tion, new leafy shoots putting out from old ones and from the protonema indefinitely, thus forming thick carpets and masses. Bog mosses often completely fill up bogs or small ponds and lakes with a dense growth, which dies below and continues to grow above as long as the conditions are favorable. These quaking bogs or "mosses," as they are sometimes called, furnish very treacherous footing unless rendered firmer by other plants. In these moss-filled bog? the water shuts off the lower strata of moss from complete disorganization, and they become modified into a coaly sub- stance called peat, which may accumulate to considerable thickness by the continued upward growth of the mass of moss. Tlie gametophyte body is differentiated into two very distinct regions : (1) the prostrate dorsiventral thallus, which is called protonema in this group, and which may be either a broad flat thallus (Fig. 104) or a set of branching filaments (Figs. 81, 102) ; (2) the erect leafy branch or gametophore (Fig. 82). This erect branch is said to be 120 PLANT STKUCTUKES radial, in contrast with the dorsiventral thallus, referring to the fact that it is exposed to similar conditions all around, and its organs are arranged about a central axis like the parts of a radiate animal. This position is much more favorable for the chlorophyll work than the dorsiventral posi- tion, as the special chlorophyll organs (leaves) can be spread out to the light freely in all directions. It should be re- marked that the gam- etophyte in all groups of plants is a thallus, doing its chlorophyll work, when it does any, in a dorsiventral position ; the only ex- ception being the ra- dial leafy branch that arises from the thal- lus of Mosses. From Mosses onward the gametophyte becomes less conspicuous, so that the prominent leafy plants of the higher groups hold no relation to the little erect leafy branch of the Mosses, which is put out by the gametophyte, and which is the best the gametophyte ever does toward getting into a bet- ter position for chlorophyll work- The leafy branch of the Mosses usually becomes inde- pendent of the thallus by putting out rhizoids at its base Fig. 102. A moss (Bryi/tn), showing base of a leafy branch (gametophore) attached to tlie protonema. and having sent out rhizoids. On the protonemal filament to the right and be- low is the young bud of another leafy branch. — MULLEK. THE GKEAT GROUPS OF BKYOPIIYTES 121 (Fig. 102), the thallus part dying. Sometimes, however, the filamentous protonema is very persistent, and gives rise to a perennial succession of leafy branches. A :P Fig. 103. Tip of leafy branch of a moss (Ftmaria), bearing a cluster of sex organs, showing an old antheridium (A), a younger one (B), some of the curious associated hairs (p), and leaf sections (/).— After Campbell. At the summit of the leafy gametophore, either upon the main axis or upon a lateral branch, the antheridia and archegonia are borne (Figs. 83, 103). Often the leaves at the summit become modified in form and arranged to form 122 PLANT STRUCTUKES a rosette, in the center of which are the sex organs. This rosette is often called the "moss flower," but it holds no relation to the flower of Seed-plants, and the phrase should not be used. A rosette may contain but one kind of sex organ (Figs, 83, 103), or it may contain both kinds, for Mosses are both dioecious and moncecious. The two prin- cipal groups are as follows : 73. Sphagnum forms. — These are large and pallid bog mosses, found abundantly in marshy ground, especially of temperate and arctic regions, and are conspicuous peat- formers (Fig. 105, A). The leaves and gametophore axis are of peculiar struc- ture to enable them to suck up and hold a large amount of wa- ter. This abundant water - storage tissue and the comparative- ly poor display of chlorophyll - contain- ing cells gives the peculiar pallid ap- pearance. They resemble the Liverworts in the broad thallus body of the gametophyte, from which the lai-ga leafy gametophore arises (Fig. 104). They also resemble Anthoceros forms in the sporogonium, the archesporium being a dome-shaped mass (Fig. 105, C). On the other hand, they resemble the true Mqsses, not only in the leafy gametophore, but also in the fact that the capsule opens at the apex by a circular lid, called the operculum (Fig. Fig. 104. Thallus body of gametophyte of Sphag- nvm, giving rise to rhizoids (r) and buds (A) which develop into the large leafy branches (gametophores).— After Campbell. THE GREAT GROUPS OF BRYOPIIYTES 123 105, D), which means a "cover" or "lid." This may serve to illustrate what is called an "intermediate" or "transition" type, Sphagnum showing characters which ally it to Anthoceros forms on the one side, and to true Mosses on the other. A peculiar feature of the sporogonium is that it has no long stalk-like" seta, as have the true Mosses, although it appears to have one. This false appearance arises from the Fig. 105. Sphagnum : A,Q. leafy branch (gametophore) bearing four mature sporo- gonia; B, archegoniuin in whose venter a young embryo sporophyte (em) is de- veloping:' C, section of a young sporogonium (sporophyte), showing the bulbous foot (spf) imbedded in the apex of the pseudopodium (ps), the capsule (A), the columella icd) cai)pcd by the dome-shaped archesporium (spo), a portion of the calyptra (ca). and the old archegonium neck {ah)\ D, branch bearing mature sporogonium and showing pseudopodium (ps), capsule (k), and operculum (d)\ E, antheridium discharging sperms; F, a single sperm, showing coiled body and two cilia.— After Scuimper. fact that the axis of the gametophore is prolonged above its leafy portion, the prolongation resembling the seta of an ordinary moss (Fig. 105, D). This prolongation is 124 PLANT STKUCTUKES called a pseudopodium, or "false stalk," and in the top of it is imbedded the foot of the sporogoninm carrying the globular capsule (Fig. 105, C). 74. True Mosses. — This immense and most highly organ- ized Bryophyte group contains the great majority of the Mosses, which are sometimes called the Bnjnm forms, to distinguish them from the Spliaynum forms. They are Fig. 106. Different stages in the development of the leafy gametophore from the pro- tonema of a common moss {Funarlay. A, the first few cells and a rhizoid (r); B, C, later stages, showing apical cell (1) and young leaves (i'); i), later stage much less magnified, showing protonemal filaments and the young gametophore {gam) — After Campbell. the representative Bryophytes, the only group vying with them being the leafy Ijiverworts, or Jungermannia forms. They grow in all conditions of moisture, from actual sub- mergence in water to dry rocks, and they also form exten- sive peat deposits in bogs. The thallus body of the gametophyte is made up of branching filaments (Figs. 81, 102), those exposed to the THE GREAT GROUPS OF BRYOPHYTES 125 light containing chlorophyll, and those in the substratum being colorless and acting as rhizoids. The leafy gameto- phores are often highly organized (Figs. 102, 106), the leaves and stems showing a certain amount of differentia- tion of tissues. It is the sporophyte, however, which shows the great- est amount of specialization (Fig. 107). The sporogonium Fig. 107. A common moss (Funaria): in the center is the leafy shoot (gametophore), with rhizoids, several leaves, and a sporogonium (sporophyte), with a long seta, capsule, and at its tip the calyptra (col): to the right a capsule with calyptra re- moved, showinK the operculum (o); to the left a young sporogonium pushing up the calyptra from the leafy shoot.— After Campbell. has a foot and a long slender seta, but the capsule is espe- cially complex. The archesporium is reduced to a small hollow cylinder (Fig. 88), the capsule wall is most elabo- rately constructed, and the columella runs through the Fig. 108. Longitiuliiuil section of moss capsule {Funcuia), showing its complex character: d, operculum; /), peristome: c, c', columel- la; s, sporogenous tissue; outside of s the complex wall consisting of layers of cells and large open spaces {h) traversed by strands of tissue.— After Goebel. Fig. 110. Sporogonia of Grimmia, from all of which the operculum has fallen, displaying the peristome teeth : A, position of the teeth when dry ; B, position when moist.— After Kerner. > Fig. 109. Partial longitudinal section through a moss cap- sule : A, younger capsule, showing wall cells (a), cells of columella (il, and sporog- enous cells {m) ; B, some- what older capsule, a and i same as before, and sm the spore mother cells. —After Goebel. THE GKEAT GROUPS OF BRYOrilYTES 127 center of the capsule to the lid-like operculum (Figs. 108, 109). When the operculum falls oil the capsule is left like an urn full of spores, and at the mouth of the urn there is usually displayed a set of slender, often very beautiful teeth (Fig. 110), converging from the circumference toward the center, and called the peristome^ meaning " about the mouth." These teeth are hygroscopic, and by bending inward and outward help to discharge the spores. CHAPTEE IX PTERIDOPHYTES (FERN PLANTS) 75. Summary from Bryophytes. — In introducing the Bryo- pliytes a summary from the Thallophytes was given (see § 60), indicating certain important things which that group has contributed to the evolution of the plant kingdom. In introducing the Pteridophytes it is well to notice certain important additions made by the Bryophytes. (1) Alternation of generations. — The great fact of alter- nating sexual (gametophyte) and sexless (sporophyte) gen- erations is first clearly expressed by the Bryophytes, although its beginnings are to be found among the Thallophytes. Each generation produces one kind of spore, from which is developed the other generation. (2) Gametophyte the chlorojih i/ll generation. — On account of this fact the food is chiefly manufactured by the gameto- phyte, which is therefore the more conspicuous generation. When a moss or a liverwort is spoken of, therefore, the gametophyte is usually referred to. (3) Gametophyte and sporophyte 7iot independent. — The sporophyte is mainly dependent upon the gametophyte for its- nutrition, and remains attached to it, being commonly called the sporogonium, and its only function is to produce spores. (4) Differentiation of thallus into stem and leaves. — This appears incompletely in the leafy Liverworts {Jwiger- mannia forms) and much more clearly in the erect and radial leafy branch (gametophore) of the Mosses. 138 PTERIDOPIiYTES 129 (5) Many-celled sex organs. — The anthericlia and the flask-shaped archegonia are very characteristic of Bryo- phytes as contrasted with Thallophytes. 76. General characters of Pteridophytes. — The name means "fern plants," and the Ferns are the most numerous and the most representative forms of the group. Associated with them, however, are the Horsetails (Scouring rushes) and the Club-mosses. By many the Pteridophytes are thought to have been derived from such Liverworts as the Antlio- ceros forms, while some think that they maj possibly have been derived directly from the Green Algce. Whatever their origin, they are very distinct from Bryophytes. One of the very important facts is the appearance of the vascular system, which means a "system of vessels," organized for conducting material through the plant body. The appearance of this system marks some such epoch in the evolution of plants as is marked in animals by the appearance of the "backbone." As animals are often grouped as "vertebrates" and "invertebrates," plants are often groujjed as "vascular plants" and "non-vascular j)lants," the former being the Pteridophytes and Spermato- phytes, the latter being the Thallophytes and Bryophytes. Pteridophytes are of great interest, therefore, as being the first vascular plants, 77. Alternation of generations. — This alternation con- tinues in the Pteridophytes, but is even more distinct than in the Bryoj)hytes, the gametophyte and sporoj)hyte be- coming independent of one another. An outline of the life history of an ordinary fern will illustrate this fact, and will serve also to point out the prominent structures. Upon the lower surface of the leaves of an ordinary fern dark spots or lines are often seen. These are found to yield spores, with Avhich the life history may be begun. When such a spore germinates it gives rise to a small, green, heart-shaped thallus, resembling a delicate and sim- ple liverwort (Fig. Ill, A). Upon this thallus antheridia 130 PLANT STEUCTUKES and archegonia appear, so that it is evidently a gameto- phyte. This gametophyte escapes ordinary attention, as it is usually very small, and lies prostrate upon the substra- tum. It has received the name prothalUum or prothallus, so that when the term prothallium is used the gametophyte of Pteridophytes is generally referred to ; j ust as when the term sporogonium is used the sporophyte of the Bryophytes is referred to. Within an archegonium borne upon this little prothallium an oospore is formed. When the oospore ger- FiG. 111. Prothallium of a common fern (Aspidimii): A, ventral surface, showing rhizoids (?'/i), antheridia (an), 'inrt archegonia {ar) ; B, ventral surface of an older gametophyte, showing rhizoids (?7i) and young sporophyte with root (w) and leaf (&).— After ScuENCK. minates it develops the large leafy plant ordinarily spoken of as "the fern," with its subterranean stem, from which roots descend, and from which large branching leaves rise above the surface of the ground (Fig. Ill, i?). It is in this complex body that the vascular system appears. No sex organs are developed upon it, but the leaves bear numer- ous sporangia full of asexual spores. This complex vascular plant, therefore, is a sporophyte, and corresponds in this life history to the sporogonium of the Bryophytes. This PTEEIDOrilYTES 131 completes the life cycle, as the asexual spores develop the prothallium again. In contrasting this life history with that of Bryophytes several important differences are discovered. The most striking one is that the sporophyte has become a large, leafy, vascular, and independent structure, not at all re- sembling its representative (the sporogonium) among the Bryophytes. Also the gametophyte is much less prominent than the gametophytes of the larger Liverworts and Mosses. If Ferns have been derived from the Liverworts, therefore, it is probable that they came from those with very simple bodies rather than from those in which the gametophyte had become large and complex. The conspicuous leafy branch of the Mosses, commonly called " the moss plant," corresponds to nothing in the Pteridophytes, the prothal- lium representing only the protonema part of the gameto- phyte of the true Mosses. The small size of the gametophyte seems to be associ- ated with the fact that the chlorophyll work has been transferred to the sporophyte, which hereafter remains the conspicuous generation. The " fern plant " of ordinary observation, therefore, is the sporophyte ; while the " moss plant '*' is a leafy branch of the gametophyte. Another important contrast indicated is that in Bryo- phytes the sporophyte is dependent upon the gametophyte for its nutrition, remaining attached to it ; Avhile in most of the Pteridophytes both generations are independent green plants, the leafy sporophyte remaining attached to the small gametophyte only while beginning its growth (Fig. Ill, B). Among the Ferns some interesting exceptions to this method of alternation have been observed. Lender certain conditions a leafy sporophyte may sprout directly from the prothallium (gametophyte) instead of from an oospore. This is called ajwgamy, meaning " without the sexual act." ;[32 PLANT STKUCTUKES Under certain other conditions prothallia are observed to sprout directly from the leafy sporophyte instead of from a spore. This is called apospoi-y, meaning " without a spore." 78. The gametophyte. — The prothallium, like a simple liverwort, is a dorsiventral body, and puts out numerous Fig. 112. Stag-horn fern (Plaiycerinm grande), an epiphytic tropical form, showing the two forms of leaves : a and b, young sterile leaves ; c, leaves bearing spo- rangia ; d, an old sterile leaf. — Caldwell. rhizoids from its ventral surface (Fig. 111). It is so thin that all the cells contain chlorophyll, and it is usually short- lived. In rare cases it becomes quite large and permanent, Fig. 113. Archegonium of Pteris at the tune of fertilization, showing tissue of gam- etophyte {A), the cells forming the neck {B), the passageway formed by the dis- organization of the canal cells (C), and the egg (D) lying exposed in the venter. — Caldwell. Fig. 114. Antheridium of PlerwiB). showing wall cells (a), opening for escape of sperm mother cells ie), escaped mother cells (c), sperms free from mother cells (6), showing spiral and multiciliate character. — Caldwell. 27 134 PLANT STRUCTURES being a conspicuo^^s object in connection with the sporo- phyte. At the bottom of the conspicuous notch in the prothal- lium is the growing point, representing the apex of the plant. This notch is always a conspicuous feature. The antheridia and arch- egonia are usually developed on the under surface of the prothallium (Fig. Ill, A), and differ from those of all Bryophytes, except the An- thoceros forms, in being sunk in the tissue of the prothal- lium and opening on the sur- PiG. 115. Development of gametophyte of Pteris: the figure to the left shows the old spore (B), the rhizoid (C), and the thallus (^4); that to the right is older, showing the same parts, and also the apical cell (Z>). — Caldwell. Fig. 116. Young gametophyte of Pteris, showing old spore wall (B), rhizoids (C), apical cell (2)), a young anther- idium (E), and an older one in which sperms have organized (i?').— Cald- well. PTEKIDOPIIYTES 135 face, more or less of the neck of the archegonium projecting (Fig. 113). The eggs are not different from those formed witliiii the areliegonia of Bryophytes, but the sperms are very different. Tlie Bryophyte sperm has a small body and two long cilia, while tlie Pteridophyte sperm has a long spirally coiled body, blunt behind and tapering to a point in front, where numerous cilia are developed (Fig. 114). It is, therefore, a large, spirally-coiled, multiciliate sperm, and is quite characteristic of all Pteridophytes excepting the Club-mosses. It is evident that a certain amount of water is necessary for fertilization — in fact, it is needed not only Fig. 117. Sections of portions of the gametophyte of Pteris, showing development of archegonium: .-1, young stage, showing cells which develop the neck (a), and the cell from which the egg cell and canal cells develop (b): B, an older stage, showing neck cells (a\ neck canal cell (6). and cell from which is derived the egg cell, and the ventral canal cell (c); C, a still older stage, showing increased num- ber of neck cells {a), two neck canal cells (A), the ventral canal cell (c), and the cell in which the egg is organized (rf). — Caldwell. by the swimming sperm, but also to cause the opening of tlie antheridium and of the archegonium neck. There seems to be a relation between the necessity of water for fertilization and a prostrate, easily moistened gametophyte. Prothallia are either monoecious or dioecious (see § 69). "Wlien the prothallia are developing (Fig. 115) the anther- Fig. 118. A fern {Aspidhim), showing three large branching leaves coming from a horizontal subterranean stem (rootstock); young leaves are al.^o shown, which show circinate vernation. The stem, young leaves, and petioles of the large leaves are thickly covered with protecting hairs. The stem gives rise to numerous small roots from its lower surface. The figure marked 3 represents the under sur- face of a portion of the leaf, showing seven sori with shield-like indusia; at ;: is represented a section through a sorus. showing the sporangia attached and pro- tected by the indusium; while at 6 is represented a single sporangium opening and discharging its spores, the heavy annulus extending along the back and over the top. — After Wossidlo. PTERIDOPUYTES 13Y idia begin to appear very early (Fig. 110), and later the archegonia (Fig. 117). If the prothalliiim is poorly nour- ished, only antheridia appear ; it needs to be well developed and nourished to develop archegonia. There seems to be a very definite relation, therefore, between nutrition and the development of the two sex organs, a fact which must be remembered in connection with the development of beterospory. 79. The sporophyte. — This complex body is differentiated into root, stem, and leaf, and is more highly organized than any plant body heretofore mentioned (Fig. 118). The development of this body and its three great working regions must be considered separately. (1) Development of embryo. — The oospore, from which the sporophyte develops, rests in the venter of the arche- gonium, which at this stage resembles a depression in the Fig. 119. Embryos of a common fern (Pteris): A, young embryo, showing direction of basal wall (/). and of second walls (11), which organize quadrants, each of which subsequently develops into foot (/), root (w). leaf (ft), and stem is): B, an older embryo, in which the four regions (lettered as in ,4) have developed further, also showing venter of archegonium (aiv), and some tissue of the prothallium (pr). —A after Kiemtz-CJeuloff; B after Hofmeister. lower surface of the prothallium (Fig. 119, B). It germi- nates at once, as in Bryophytes, not being a resting spore as in Green Algse. The resting stage, as in the Bryophytes, 138 FLANT STRUCTURES is in connection with the asexual spores, which may be kept for a long time and then germinated. The first step in germination is for the oospore to di- vide into two cells, forming a two-celled embryo. In the ordinary Ferns this first dividing wall is at right angles to the surface of the prothallium, and is called the basal wall (Fig. 119, A). One of the two cells, therefore, is anterior (toward the notch of the prothallium), and the other is posterior. The two cells next divide by forming walls at right angles to the basal wall, and a four-celled embryo is the result. This is called the "quadrant stage" of the em- bryo, as each one of the four cells is like the quadrant of a sphere. With the appearance of the quadrant, four body regions are organized, each cell by its subsequent divisions giving rise to a distinct working region (Fig. 119, A). Two of the cells are inner (away from the substratum) ; also one of the inner and one of the outer (toward the substratum) cells are anterior ; while the two other inner and outer cells are posterior. The anterior outer cell develops the first leaf of the embryo, generally called the cotyledon (Fig. 119, b) ; the anterior inner cell develops the stem (Fig. 119, s) ; the pos- terior outer cell develops the first (primary) root (Fig. 119, w) ; the posterior inner cell develops a special organ for the use of the embryo, called the foot (Fig. 119, /). The foot remains in close contact with the prothallium and absorbs nourishment from it for the young embryo. When the young sporophyte has developed enough to become in- dependent the foot disappears. It is therefore spoken of as a temporary organ of the embryo. It is necessary for the leaf to emerge from beneath the prothallium, and it may be seen usually curving upward through the notcli. The other parts remain subterranean. (3) The root. — The primary root organized by one of the quadrants of the embryo is a temporary affair (Figs. PTERIDOPIIYTES 139 111, 119), as it is in an unfavorable position in reference to the dorsiventral stem, Avhich puts out a series of more favor- ably placed secondary roots into the soil (Fig. 118). The mature leafy sporophyte, therefore, has neither foot nor primary root, the product of two of the quadrants of the embryo having disappeared. The secondary roots put out by the stem are small, and do not organize an extensive system, but they are interest- ing as representing the first appearance of true roots, which therefore come in with the vascular system. In the lower groups the root function of absorption is not assumed by any special organ, unless rhizoids sometimes absorb ; but true roots are complex in structure and contain vessels. (3) The stem. — In most of the Ferns the stem is sub- terranean and dorsiventral (Fig. 118), but in the "tree ferns " of the tropics it forms an erect, aerial shaft bearing a crown of leaves (Fig. 120). In the other groups of Pteri- dophytes there are also aerial stems, both erect and pros- trate. The stem is complex in structure, the cells being organized into different "tissue systems," prominent among which is the vascular system. These tissue systems of vas- cular plants are described in Chapter XV. The appearance of the vascular system in connection with the leafy sporophyte is worthy of note. The leaves are special organs for chlorophyll work, and must receive the raw material from air and soil or water. The leaves of the moss gametophyte are very small and simple affairs, and can be supplied with material by using very little ap- paratus. In the leafy sporophyte, however, the leaves are very prominent structures, capable of doing a great deal of work. To such working structures material must be brought rapidly in quantity, and manufactured food ma- terial must be carried away, and therefore a special con- ducting apparatus is needed. This is supplied by the vas- cular system. These vessels extend continuously from root- tips, through the stem, and out into the leaves, where they FiQ. 120. A group of tropical plants. To the left of the center is a tree fern, with its slender columnar stem and crown of large leaves. The large-leaved plants to the right are bananas (Monocotyledons).— From " Plant Eelations." PTEKIDOPHYTES 141 are spoken of as ''leaf veins." Large working leaves and a vascular system, therefore, belong together and appear together; and the vascular plants are also the plants with leafy sporophytes. (4) The leaf. — Leaves are devices for spreading out green tissue to the light, and in the Ferns they are usually large. There is a stalk-like portion (petiole) which rises from the subterranean stem, and a broad expanded portion (blade) exposed to the light and air (Fig. 118). In Ferns the blade is usually much branched, being cut up into segments of various sizes and forms. The essential structure consists of an expansion of green tissue (mesophi/Il), through which strands of the vascular system (veins) branch, forming a supporting framework, and over all a compact layer of protecting cells (epidermis). A surface view of the epidermis shows that it is pierced by numer- ous peculiar pores, called stomata, meaning " mouths." The surface view of a stoma shows two crescentic cells (guard cells) in contact at the ends and leaving be- tween them a lens-shaped opening (Fig. 121). A cross-section through a leaf gives a good view of the three regions (Fig. 122). Above and below is the col- orless epidermis, pierced here and there by stomata ; between the epidermal lay- ers the cells of the mesophyll are packed ; and among the mesophyll cells there may be seen here and there the cut ends of the veins. The leaf is usually a dorsiventral Fig. 121. Some epidermal cells froin leaf of Pteris, showing the inter- locking walLs and three stomata, the guard cells containing chloroplasts. — Land. 142 PLANT STRUCTURES organ, its two surfaces being differently related to light. To this different relation the mesophyll cells respond in their arrangement. Those in contact with the upper epi- dermis become elongated and set endwise close together, forming the palisade tissue; those below are loosely ar- Fio. 122. Cross-seclion through a portion of the leaf of Pteris, showing the heavy- walled epidermis above and below, two stomata in the lower epidermis (one on each side of the center) opening into intercellular passages, the mesophyll cells containing chloroplasts, the upper row arranged in palisade fashion, the other cells loosely arranged (spongy mesophyll) and leaving large intercellular passages, and in the center a section of a veinlet (vascular bundle), the xylem being repre- sented by the central group of heavy-walled cells. — Land. ranged, leaving numerous intercellular spaces, forming the spongy tissue. These spaces form a system of inter- cellular passageways among the working mesophyll cells, putting them into communication with the outer air through the stomata. The freedom of this communication PTERIDOPHYTES 143 is regulated by the guard cells of the stomata, which come together or shrink apart as occasion requires, thus dimin- ishing or enlarging the opening between them. The sto- mata have well been called "automatic gateways " to the system of intercellular passageways. One of the peculiarities of ordinary fern leaves is that the vein system branches dichotomously, the forking veins being very conspicuous (Figs. 133-126). Another fern habit is that the leaves in expanding seem to unroll from the base, as though they had been rolled from the apex downward, the apex being in the center of the roll (Fig. 118). This habit is spoken of as circinate, from a word meaning '^ circle" or "coil," and circinate leaves when unrolling have a crozier-like tip. The arrangement of leaves in bud is called vernation ("spring condition"), and therefore the Ferns are said to have circinate verna- tion. The combination of dichotomous venation and cir- cinate vernation is very characteristic of Ferns. 80. Sporangia. — Among Thallophytes sporangia are usu- ally simple, mostly consisting of a single mother cell ; among Bryophytes simple sporangia do not exist, and in connec- tion with the usually complex capsule of the sporogonium the name is dropped ; but among Pteridophytes distinct sporangia again appear. They are not simple mother cells, but many-celled bodies. Their structure varies in different groups of Pteridophytes, but those of ordinary Ferns may be taken as an illustration. The sporangia are borne by the leaves, generally upon the under surface, and are usually closely associated with the veins and organized into groups of definite form, known as sori. A sorus may be round or elongated, and is usually covered by a delicate flap {indu^ium) which arises from the epidermis (Figs. 118, 123, 124). Occasionally the sori are extended along the under surface of the margin of the leaf, as in maidenhair fern {Adiantnm), and the common brake {Pteris), in which case they are protected by the inrolled 1^ -m»oM44 Fig. 123. Fragrant shield fern (A«/wc?- ium frar/rans), showing general habit, and to the left (a) the under surface of a leaflet bearing sori covered by shield-like indusia.— After Marion Satterlee. Fig. 124. The bladder fern (Cystoiiteris bulb- ifera), showing general habit, and to the right (a) the under surface of a leaflet, showing the dichotomous venation, and five sori protected by pouch-like indusia. —After Marion Satterlee. PTERIDOPHYTES 145 margin (Figs. 125, 126), which may be called a "false in- dnsium." It is evident that such leaves are doing two distinct kinds of work — clilorophyll work and sj)ore formation. This is true of most of the ordinary Ferns, but some of them show a tendency to di- vide the work. Certain leaves, or certain leaf-branches, pro- duce spores and do no chloro- phyll work, while others do chlorophyll work and produce no spores. This differentia- tion in the leaves or leaf-re- gions is indicated by appro- priate names. Those leaves which jDroduce only spores are called sporo2JhijlU, meaning "spore leaves," while the leaf branches thus set apart are called sporophyll branches. Those leaves which only do chlorophyll work are called /o- liage leaves ; and such branch- es are foliage branches. As sporophylls are not called upon for chlorophyll work they often become much modified, being much more compact, and not at all resembling the foliage leaves. Such a differentiation may be seen in the ostrich fern and sensitive fern ( Onoclea) (Figs. 127, 128), the climbing fern {Lygodium), the royal fern {Osmunda), the moonwort {Botrychium) (Fig. 129), and the adder's tongue {Ophioglossiim) (Fig. 130). An ordinary fern sporangium consists of a slender stalk and a bulbous top wliich is the spore case (Fig. 118, 6). This case has a delicate wall formed of a single layer of cells, and extending around it from the stalk and nearly to Fig. 12.5. Leaflets of two common ferns : A, the common brake (Pteris) ; B, maidenhair (Adian- finii) ; both showing sori borne at the margin and protected by the infolded margin, which thus forms a false indusium. — Cald- well. 146 PLANT STEUCTUKES the stalk again, like a meridian line about a globe, is a row of peculiar cells with thick walls, forming a heavy ring, called the mimdus. The annulus is like a bent spring, and when the delicate wall becomes yielding the spring straightens violently, the wall is torn, and in the recoil the spores are discharged with considerable force (Fig. 131). This dis- FiG. 126.— The purple cliflf brake (Pelhva atropui-purea), showing general habit, and at a a single leaflet showing the dichotomons venation and the infolded margin covering the sori. — After Marion Satterlee. charge of fern spores may be seen by placing some sporangia upon a moist slide, and under a low power watching them as they dry and burst. Within this sporangium the archesporium (see § 66) consists of a single cell, which by division finally produces PTERIDOniYTES 147 numerous mother cells, in each of which a tetrad of spores is formed. The disorganization of the walls of the mother Fig. 127 The ostrich (eTn(Onoclea stnithiopteris), showing differentiation of foliage leaf (a) and sporophyll (6).— After Marion Satterlee. cells sets the spores free in the cavity of the sporangium, and ready for discharge. Fig. 128. The sensitive lern (Otioclea sensibilin). sliowing differentiation of foliage leaves and sporophylls.— From "Field, Forest, and Wayside Flowers." PTEEIDOPIIYTES 149 Among the Bryophytes the sporogenous tissue appears very early in the development of the sporogonium, the pro- duction of spores being its only function ; also there is a ^ Pig. 129. A nioonwort (BotnjcM- ttm), showing the leaf differen- tiated into foliage and sporophyll branches. — After Strasburger. 28 Fig. 130. The adder's tongue {Ophioglossum ■vulgatuni), showing two leaves, each with a foliage branch and a much longer sporophyll branch. — After Marion Sat- TERLEE. 150 PLANT STRUCT ORES tendency to restrict the sporogenous tissue and increase the sterile tissue. It will be observed that with the introduc- tion of the leafy sporophyte among the Pteridophytes the sporangia appear much later in its development, sometimes not appearing for several years, as though they are of "^^s ^ %;j^ ^ .^^^^ Fig. 131. A series showing the dehiscence of a fern sporangium, the rupture of the wall, the straightening and bending back of the annulus, and the recoil.— After Atkinson. secondary importance as compared with chlorophyll work ; and that the sporogenous tissue is far more restricted, the sporangia forming a very small part of the bulk of the sporophyte body. PTERIDOPHYTES 151 81. Heterospory. — This phenomenon appears first among Pteridophytes, but it is not characteristic of them, being en- tirely absent from the true Ferns, which far outnumber all other Pteridophytes. Its cliief interest lies in the fact that it is universal among the Spermatophytes, and that it rep- resents the change which leads to the appearance of that high group. It is impossible to understand the greatest group of plants, therefore, without knowing something about heterospory. As it begins in simple fashion among Pteridophytes, and is probably the greatest contribution they have made to the evolution of the plant kingdom, unless it be the leafy sporophyte, it is best explained here. In the ordinary Ferns all the spores in the sporangia are alike, and when they germinate each spore produces a prothallium upon which both antheridia and archegonia appear. It has been remarked, however, that some pro- thallia are dioecious — that is, some bear only antheridia and others bear only archegonia. In this case it is evident that the spores in the sporangium, although they may ap- pear alike, produce different kinds of prothallia, which may be called male and female, as each is distinguished by the sex organ which it produces. As archegonia are only produced by well-nourished prothallia, it seems fair to sup- pose that the larger spores will produce female prothallia, and the smaller ones male prothallia. This condition of things seems to have developed finally into a permanent and decided difference in the size of the spores, some being quite small and others relatively large, the small ones producing male gametophytes (prothallia with antheridia), and the large ones female gametophytes (prothallia with archegonia). When asexual spores differ thus permanently in size, and give rise to gametophytes of different sexes, we have the condition called heterospory (''spores different"), and such plants are called heterospo- rous (Fig. 139). In contrast with heterosporous plants, those in which the asexual spores appear alike are called homos- 152 PLANT STRUCTUEES porous, or sometimes isosporous, both terms meaning "spores similar." The corresponding noun form is honios- pory or isospory. Bryophytes and most Pteridophytes are homosporous, while some Pteridophytes and all Spermato- phytes are heterosporous. It is convenient to distinguish by suitable names the two kinds of asexual spores produced by the sporangia of heterosporous plants (Fig. 139). The large ones are called megaspores, or by some writers macrospores, both terms meaning " large spores"; the small ones are called m/cro- spores, or "small spores." It should be remembered that megaspores always produce female gametophytes, and mi- crospores male gametophytes. This differentiation does not end with the spores, but soon involves the sporangia (Fig. 139). Some sporangia produce only megaspores, and are called tnegasporatigia ; others produce only microspores, and are called microspo- rangia. It is important to note that while microsporangia usually produce numerous microspores, the megasporangia produce much fewer megaspores, the tendency being to diminish the number and increase the size, until finally there are megasporangia which produce but a single large functioning megaspore. The differentiation goes still further. If the sporangia are born upon sporophylls, the sporophylls themselves may differentiate, some bearing only megasporangia, and others only microsporangia, the former being called megasporo- phylls, the latter microsporopliylls. In such a case the sequence is as follows : megasporophylls produce megaspo- rangia, which produce megaspores, which in germination produce the female gametophytes (prothallia with archego- nia) ; while the microsporophylls produce microsporangia, which produce microspores, which in germination produce male gametophytes (prothallia with antheridia). A formula may indicate the life history of a heteros- porous plant. The formula of homosporous plants with PTERIDOPHYTES I53 alternation of generations (Bryophytes and most Pterido- phytes) was given as follows (§ 63) : G=g> 0— S— 0— G=:8> 0— S— 0— Giz8> 0— S, etc. In the case of heterosporous plants (some Pteridophytes and all Spermatophytes) it would be modified as follows : g=8> o-S=8=^=8> 0— Si=8:=S=8> 0— S, etc. In this case two gametophytes are involved, one pro- ducing a sperm, the other an egg, which fuse and form the oospore, which in germination produces the sporophyte, which produces two kinds of asexual spores (megaspores and microspores), which in germination produce the two gametophytes again. One additional fact connected with heterospory should be mentioned, and that is the great reduction of the gam- etophyte. In the homosporous ferns the spore develops a small but free and independent prothallium which pro- duces both sex organs. When in heterosporous plants this work of producing sex organs is divided between two gam- etophytes they become very much reduced in size and lose their freedom and independence. They are so small that they do not escape entirely, if at all, from the embrace of the spores which produce them, and are mainly dependent for their nourishment upon the food stored up in the spores (Figs. 140, 141). As the spore is produced by the sporo- phyte, heterospory brings about a condition in which the gametophyte is dependent upon the sporoi3hyte, an exact reversal of the condition in Bryophytes. The relative importance of the gametophyte and the sporophyte throughout the plant kingdom may be roughly indicated by the accompanying diagram, in which the shaded part of the parallelogram represents the gameto- phyte and the unshaded part the sporophyte. Among the ]^54 PLANT STKUCTUEES lowest plants the gametophyte is represented by the whole plant structure. When the sporophyte first appears it is dependent upon the gametophyte (some Thallophytes and the Bryophytes), and is relatively inconspicuous. Later the sporophyte becomes independent (most Pteridophytes), the gametophyte being relatively inconspicuous. Finally (heterosporous Pteridophytes) the gametophyte becomes dependent upon the sporophyte, and in Spermatophytes is so inconspicuous and concealed that it is only observed by means of laboratory appliances, while the sporophyte is the whole plant of ordinary observation. CHAPTER X THE GREAT GROUPS OF PTERIDOPHYTES 82. The great groups.— At least three independent lines of Pteridopliytes are recognized : (1) Filicales (Ferns), (2) Equisetales (Scouring rushes, Horsetails), and (3) Ly- copodiahs (Club-mosses). The Ferns are much the most abundant, the Club-mosses are represented by a few hun- dred forms, while the Horsetails include only about twenty- five species. These three great groups are so unlike that they hardly seem to belong together in the same division of the plant kingdom. Filicales {Ferns) 83. General characters.— The Ferns were used in the preceding chapter as types of Pteridophytes, so that little need be added. They well deserve to stand as types, as they contain about four thousand of the four thousand five hundred species belonging to Pteridophytes. Although found in considerable numbers in temperate regions, their chief display is in the tropics, where they form a striking and characteristic feature of the vegetation. In the trop- ics not only are great masses of the low forms to be seen, from those with delicate and filmy moss like leaves to those with huge leaves, but also tree forms with cylindrical trunks encased by the rough remnants of fallen leaves and sometimes rising to a height of thirty-five to forty-five feet, with a great crown of leaves fifteen to twenty feet long (Fig. 120). 155 THE GKEAT GROUPS OF PTERIDOPHYTES I57 There are also epiphytic forms (air plants) — that is, those which perch " upon other plants " but derive no nourishment from them (Fig. 112). This habit belongs chiefly to the warm and moist tropics, where the plants can absorb sufficient moisture from the air without send- ing roots into the soil. In this way many of the tropical ferns are found growing upon living and dead trees and other plants. In the temperate regions the chief ejai- phytes are Lichens, Liverworts, and Mosses, the Ferns be- ing chiefly found in moist woods and ravines (Fig. 133), although a number grow in comparatively dry and exposed situations, sometimes covering extensive areas, as the com- mon brake (Pteris) (Fig. 125). The Filicales differ from the other groups of Pterido- phytes chiefly in having few large leaves, which do chloro- phyll work and bear sporangia. In a few of them there is a differentiation of functions in foliage branches and sporo- phyll branches (Figs. 127-130), but even this is excep- tional. Another distinction is that the stems are un- branched. 84. Origin of sporangia. — An important feature in the Ferns is the origin of the sporangia. In some of them a sporangium is developed from a single epidermal cell of the leaf, and is an entirely superficial and generally stalked affair (Fig. 118, 5) ; in others the sporangium in its devel- opment involves several epidermal and deeper cells of the leaf, and is more or less of an imbedded affair. In the first case the ferns are said to be leptosporangiate ; in the sec- ond case they are euspoi'angiate. The leptosporangiate Ferns are overwhelmingly abun- dant as compared with the Eusporangiates. Back in the Coal-measures, however, there was an abundant fern vege- tation which was probably all eusporangiate. The Lep- tosporangiates seem to be the modern Ferns, the once abundant Eusporangiates being represented now in the temperate regions only by such forms as moonwort {Bo- 158 PLANT STKUCTUKES trycliium) (Fig. 129) and adder's tongue (OpMoglossiim) (Fig. 130). It is important to note, however, that the Horsetails and Club-mosses are Eusporangiates, as well as all the Seed-plants. Another small but interesting group of Ferns includes the ''Water-ferns," floating forms or sometimes on muddy flats. The common Marsilia may be taken as a type (Fig. 133). The slender creeping stem sends down numerous roots into the mucky soil, and at intervals gives rise to a comparatively large leaf. This leaf has a long erect petiole and a blade of four spread- r^ r^ Fig. 133.— a water-fern {Marsilia), showing horizontal stem, with descending roots, and ascend- ing leaves ; a, a young leaf showing circinate vernation ; s,s, sporophy 1 1 branches ( " spo- rocarps "). — After Bischoff. A ' ' ' J? Fig. 134. One of the floating water-ferns (Sal- vinia), showing side view (^4) and view from above (B). The dangling root-like processes are the modified submerged leaves. In A, near the top of the cluster of submerged leaves, some sporophyll branches ("sporo- carps") may be seen. — After Bischoff. ing wedge-shaped leaflets like a " four-leaved clover. " The dichotomous venation and circinate vernation at once sug- gest the fern alliance. From near the base of the petiole THE GREAT GROUPS OF PTERIDOPHYTES 159 another leaf branch arises, in which the blade is modified as a sporophyll. In this case the sporophyll incloses the sporangia and becomes hard and nut-like. Another com- mon form is the floating Salvinia (Fig. 134). The chief interest lies in the fact that the water-ferns are heteros- porous. As they are leptosporangiate they are thought to have been derived from the ordinary leptosporangiate Ferns, which are homosporous. Three fern groups are thus outlined : (1) homosporous- eusporangiate forms, now almost extinct ; (2) homosporous- leptosporangiate forms, the great overwhelming modern group, not only of Filicales but also of Pteridophytes, well called true Ferns, and thought to be derived from the pre- ceding group ; and (3) heterosporous-leptosporangiate forms, the water-ferns, thought to be derived from the pre- ceding group. Equisetales ( Horsetails or Scouring rushes) 85. General characters. — The twenty-five forms now rep- resenting this great group belong to a single genus {Equise- tum, meaning "horsetail"), but they are but the linger- ing remnants of an abundant flora which lived in the time of the Coal-measures, and helped to form the forest vegeta- tion. The living forms are small and inconspicuous, but very characteristic in appearance. They grow in moist or dry places, sometimes in great abundance (Fig. 135). The stem is slender and conspicuously Jointed, the joints separating easily ; it is also green and fluted with small longitudinal ridges ; and there is such an abundant deposit of silica in the epidermis that the plants feel rough. This last property suggested its former use in scouring, and its name " scouring rush." At each joint is a sheath of minute leaves, more or less coalesced, the individual leaves some- times being indicated only by minute teeth. This arrange- ment of leaves in a circle about the joint is called the cyclic Fig. 135. Equisetian arvense, a common horsetail: 1. three fertile shoots rising from the dorsiventral stem, showing the cycles of coalesced scale-leaves at the joints and the terminal strobili with numerous sporophylls, that at a being mature; 2, a sterile shoot from the same stem, showing branching; 3, a single peltate sporo- phyll bearing sporangia; A, view of sporophyll from beneath, showing dehiscence of sporangia; 5, 6, 7. spores, showing the unwinding of the outer coat, which aids in dispersal.— After Wossidlo. THE GREAT GROUPS OF PTERIDOPIIYTES IGl arrangement, or sometimes the luliorUd arrangement, each such set of leaves being called a cycle or a ivhorl. These leaves contain no chlorophyll and have evidently abandoned chlorophyll work, which is carried on by the green stem. Such leaves are known as scales, to distinguish them from foliage leaves. The stem is either simple or profusely branched (Fig. 135). 86. The strobilus. — One of the distinguishing characters of the group is that chlorophyll-work and spore-formation are completely difEerentiated. Although the foliage leaves Dioecious gametophytes of Eqmsetvm ; A, the female gametophyte, show- branching, i-hizoids. and an archegonium (ar); B, the male gametophyte, IPriflifl i i^ \ A f f Pr P iTVT-P'RWT T. uig orancning, rnizoias. ana an arcnegonium (ar) showing several antheridia ( 6 ).— After Campbell. are reduced to scales, and the chlorophyll-work is done by the stem, there are well-organized sporophylls. The sporo- phylls are grouped close together at the end of the stem in a compact conical cluster which is called a sfrohihn^, the Latin name for "pine cone," which this cluster of sporo- phylls resembles (Fig. 135). Each sporophyll consists of a stalk-like portion and a shield-like {peltate) top. Beneath the shield hang the 162 PLANT STKUCTURES sporangia, which produce spores of but one kind, hence these plants are homosporous ; and as the sporangia origi- nate in eusporangiate fashion, Eqvisetum has the homospo- rous-eusporangiate combination shown by one of the Fern groups. It is interesting to know, however, that some of the ancient, more highly organized members of this group were heterosporous, and that the present forms have dioecious gametophytes (Fig. 136). Lycopodiales {Cluh-mosses) 87. General characters. — This group is now represented by about five hundred species, most of which belong to the two genera Lt/copodiiim and Selagmella, the latter being much the larger genus. The plants have slender, branching, prostrate, or erect stems completely clothed with small foliage leaves, having a general moss-like appearance (Fig. 137). Often the erect branches are terminated by conspicuous conical or cylindrical strobili, which are the " clubs " that enter into the name " Club- mosses." There is also a certain kind of resemblance to miniature pines, so that the name " Ground-pines " is sometimes used. Lycopodiales were once much more abundant than now, and more highly organized, forming a conspicuous part of the forest vegetation of the Coal-measures. One of the distinguishing marks of the group is that the sperm does not resemble that of the other Pteridophytes, but is of the Bryophyte type (Fig. 140, F). That is, it consists of a small body with two cilia, instead of a large sj^irally coiled body with many cilia. Another distinguish- ing character is that there is but a single sporangium pro- duced by each sporophyll (Fig. 137). This is in marked contrast with the Filicales, whose leaves bear very numer- ous sporangia, and with the Equisetales, whose sporophylls bear several sporangia. THE GKEAT GROCPS OF PTERIDOPllYTES 163 Fie. 137. A common chib-moss (LycopodXmn clavatinn): 1, the whole plant, showing horizontal stem giving rise to roots and to erect branches bearing strobili; 2, a single sporophyll with its sporangium; 3, spores, much magnified.— After Wos- 8IDLO. 88. Lycopodium. — This genus contains fewer forms than the other, but they are larger and coarser and more charac- teristic of the temperate regions, being the ordinary Club- mosses (Fig, 137). They also more commonly display conspicuous and distinct strobili, although there is every 164 PLANT STRUCTUKES gradation between ordinary foliage leaves and distinct sporophylls. The sporangia are borne either by distinct sporophylls or by the ordinary foliage leaves near the summit of the stem. At the base of each of these leaves, or sporophylls, on the upper side, is a single sporangium (Fig. 137). The sporangia are eusporangiate in origin, and as the spores are all alike, Lycopodium has the same homosporous-eusporan- giate combination noted in Equisetales and in one of the groups of Filicales. 89. Selaginella. — This large genus contains the smaller, more delicate Club-mosses, often being called the " little Club-mosses." They are especially displayed in the trop- Fia. 138. Selaginella, showing general epraylike habit, and dangling leafless stems which strike root (rAisopAores).— From " Plant Relations." ics, and are common in greenhouses as delicate, mossy, decorative plants (Fig. 138). In general the sporophylls are not different from the ordinary leaves (Fig. 139), but sometimes they are modified, though not so distinct as in certain species of Lycopodium. THE GREAT GKOUrS OF rTEKIDOl'HYTES 165 The solitary sporangium appears in the axils (upper angles formed by tlie leaves with the stem) of the leaves and sporophylls, but arise from the stem instead of the Pig. 139. Selaginella Martensii : A, branch bearing strobili; B, a microsporophyll with a microsporangiiim, showing microspores through a rupture in the wall; C, a megasporopliyll with a megasporangium ; D, megaspores : E, microspores.— GOLDBERGER. 29 166 PLANT STRUCTDRES leaf (Fig. 139). This is important as showing that sporan- gia may be produced by stems as well as by leaves, those being produced by leaves being called foliar, and those by stem cauUne. The most important fact in connection with Selaginella, however, is that it is heterosporous. Megasporangia, each usually containing but four megaspores, are found in the axils of a few of the lower leaves of the strobilus, and more numerous microsporangia occur in the upjDer axils, con- taining very many microspores (Fig. 139). The character of the gametophytes of heterosporous Pteridophytes may be well illustrated by those of Selaginella. The microspore germinates and forms a male gameto- phyte so small that it is entirely included within the spore IV j; Fig. 140. Male gametophyte of Selaginella : in each case p is the prothallial cell, w the wall cells of the antheridium, s the sperm tissue: F, the biciliate sperms. — After Belajeff. wall (Fig. 140). A single small cell is all that represents the ordinary cells of the prothallium, while all the rest is an antheridium, consisting of a wall of a few cells sur- rounding numerous sperm mother cells. In the presence THE GREAT GROUPS OF PTERIDOPHYTES 167 of water the antheridium wall breaks down, as also do the walls of the mother cells, and the small biciliate sperms are set free. The much larger megaspores germinate and become filled with a mass of numerous nutritive cells, representing the ordinary cells of a prothallium (Fig. 141). The spore wall is broken by this growing prothallium, a part of which thus protrudes and becomes exposed, although the main part of it is still invested by the old megaspore wall. In this exposed portion of the female gameto- |i \] ^ ^^ phyte the archegonia appear, and thus be- come accessible to the sperms. In the case of Isoetes (see § 90) the reduction of the female gametophyte is even greater, as it does not project from the megaspore wall at all, and the archegonia are made accessible through cracks in the wall immediately over them. The embryo of Se- laginella is also impor- tant to consider. Be- ginning its development in the venter of the archegonium, it first lies upon the exposed margin of the prothallium, while the mass of nutritive cells lie deep within the mega- spore (Fig. 141, e7nh^, emh^). It first develops an elongated cell, or row of cells, which thrusts the embryo cell deeper among the nutritive cells. This cell or row of cells, formed by the embryo to place the real embryo cell in better rela- spm Fio. 141. YemaXe gn.n\eto\^\\yte oi a. Selagiriella : spm, wall of megaspore ; pr. gametophyte ; ar, an archegonium ; einb^ and emb^. em- bryo .s])orophyte8 ; et. suspensors ; the gam- etophyte has developed a few rhizoids. — After Pfeffer. 1G8 PLANT STKUCTUKES tion to its food supply, is called the suspe?isor, and is a temporary organ of the embryo (Figs. 141, 142, et). At the end of the snspensor the real embryo develops, and when its regions become organized it shows the following parts : (1) a large foot buried among the nutritive cells of the prothallium and absorbing nourishment ; (2) a root stretching out toward the substratum ; (3) a stem extend- FiG. 142. Embryo of Selaginella removed from the gainetophyte, showing snspensor (et), root-tip («•), foot (/), cotyledons (W), stem-lip (st), and ligules (lig).— After Pfeffer. ing in the other direction, and bearing Just behind its tip (4) a pair of opposite leaves (cotyledons) (Fig. 142). As the sporangia of Selaginella are eusporangiate, this genus has the heterosporous-eusporangiate combination — a combination not mentioned heretofore, and being of special interest as it is the combination which belongs to all the Spermatophytes. For this and other reasons, Selaginella is one of the Pteridophyte forms which has attracted special attention, as possibly representing one of the an- cestral forms of the Seed-plants. THE GREAT GROUPS OF PTERIDOPIIYTES 109 90. Isoetes. — This little group of aquatic plants, known as "quilhvorts," is very puzzling as to its relationships among Pteridophytes. By some it is put with the Ferns, forming a distinct division of Filicales ; by others it is put Fig. 143. A common quillwort (Isoetes lacns- tris), showing cluster of roots dichoto- mously branching, and cluster of leaves each enlarged at base and inclosing a sin- gle sporangium.— After Schenck. Fig. 144. Sperm of Isoetes, show- ing spiral body and seven long cilia arising from the beak.— After Belajefp. with the Club-mosses, and is associated with Selaginella. It resembles a bunch of fine grass growing in shoal water or in mud, but the leaves enlarge at the base and overlap one another and the very short tuberous stem (Fig. 143). Within each enlarged leaf base a single sporangium is formed, and the cluster contains both megasporangia and microsporangia. The sporangia are eusporangiate, and therefore Isoetes shares with Selaginella the distinction of l^Q PLANT STRUCTURES having the heterosporous-eusporangiate combination, which is a feature of the Seed-plants. The embryo is also peculiar, and is so suggestive of the embryo of the Monocotyledons (see § 114) among Seed- plants that some regard it as possibly representing the ancestral forms of that group of Spermatophytes. The peculiarity lies in the fact that at one end of the axis of the embryo is a root, and at the other the first leaf (cotyledon), while the stem tip rises as a lateral outgrowth. This is exactly the distinctive feature of the embryo of Monocoty- ledons. The greatest obstacle in the way of associating these quillworts with the Club-mosses is the fact that their sperms are of the large and spirally coiled multiciliate type which belongs to Filicales and Equisetales (Fig. 144), and not at all the small biciliate type which characterizes the Club- mosses (Fig. 140). To sum up, the short unbranched stem with comparatively few large leaves, and the coiled multi- ciliate sperm, suggest Filicales ; while the solitary spo- rangia and the heterosporous-eusporangiate character sug- gest SeJagineUa. CHAPTEE XI SPERMATOPHYTES : GYMNOSPERMS 91. Summary from Pteridophytes. — In considering the important contributions of Pteridophytes to the evolution of the plant kingdom the following seem worthy of note : (1) Prominence of sporophyte and development of vascu- lar system. — This prominence is associated with the display of leaves for chlorophyll work, and the leaves necessitate the work of conduction, which is arranged for by the vas- cular system. This fact is true of the whole group. (2) Differentiation of sporopJtylls.— The appearance of sporophylls as distinct from foliage leaves, and their or- ganization into the cluster known as the strobilus, are facts of prime importance. This differentiation appears more or less in all the great groups, but the strobilus is distinct only in Horsetails and Club-mosses. (3) Introduction of heterospory and reduction of garnet o- phytes. — Heterospory appears independently in all of the three great groups — in the water-ferns among the Fili- cales, in the ancient horsetails among the Equisetales, and in Selaginella and Isoetes among Lycopodiales. All the other Pteridophytes, and therefore the great majority of them, are homosporous. The importance of the appear- ance of heterospory lies in the fact that it leads to the development of Spermatophytes, and associated with it is a great reduction of the gametophytes, which project little, if at all, from the spores which produce them. 92. Summary of the four groups.— It may be well in this connection to give certain prominent characters which will 171 1^2 PLANT STRUCTURES serve to distinguish the four great groups of plants. It must not be supposed that these are the only characters, or even the most important ones in every case, but they are convenient for our purpose. Two characters are given for each of the first three groups — one a positive, character which belongs to it, the other a negative character which distinguishes it from the group above, and becomes the positive character of that group. (1) Tliallojjhytes. — Thallus body, but no archegonia. (2) Bryoiiliytes. — Archegonia, but no vascular system. (3) Pteridophytes. — Vascular system, but no seeds. (4) Sjmrmatojjltytes. — Seeds. 93. General characters of Spermatophytes. — This is the greatest group of plants in rank and in display. So con- spicuous are they, and so much do they enter into our experience, that they have often been studied as "botany," to the exclusion of the other groups. The lower groups are not meiely necessary to fill out any general view of the plant kingdom, but they are absolutely essential to an understanding of the structures of the highest group. This great dominant group has received a variety of names. Sometimes they are called Antliopliytes, meaning "Flowering plants," with the idea that they are distin- guished by the production of "flowers." A flower is diffi- cult to define, but in the popular sense all Spermatophytes do not produce flowers, while in another sense the strobilus of Pteridophytes is a flower. Hence the flower does not accurately limit the group, and the name Anthophytes is not in general use. Much more commonly the group is called Phanerogams (sometimes corrupted into Phgenogams or even Phenogams), meaning " evident sexual reproduc- tion." At the time this name was proposed all the other groups were called Cryptogams, meaning "hidden sexual reproduction." It is a curious fact that the names ought to have been reversed, for sexual reproduction is much more evident in Cryptogams than in Phanerogams, the mistake SPKRMATOPIIYTES: GYMNOSPEKMS 1^3 arising from the fact that what were supposed to be sexual organs in Phanerogams have proved not to be such. The name Phanerogam, therefore, is being generally abandoned ; but the name Cryptogam is a useful one when the lower groups are to be referred to ; and the Pteridophytes are still very frequently called the Vascular Cryptogams. The most distinguishing mark of the group seems to be the production of seeds, and hence the name Spermatojjhytes, or " Seed-plants," is coming into general use. The seed can be better defined after its development has been described, but it results from the fact that in this group the single megaspore is never discharged from its megasporangium, but germinates just where it is devel- oped. The great fact connected with the group, therefore, is the retention of the megaspore, which results in a seed. The full meaning of this will appear later. There are two very independent lines of Seed-plants, the Gymnospenns and the Angiosperfiis. The first name means "naked seeds," referring to the fact that the seeds are always exposed ; the second means " inclosed seeds," as the seeds are inclosed in a seed vessel. Gymnosperms 94. General characters. — The most familiar Gymnosperms in temperate regions are the pines, spruces, hemlocks, cedars, etc., the group so commonly called ''evergreens." It is an ancient tree group, for its representatives were associated with the giant club-mosses and horsetails in the forest vegetation of the Coal-measures. Only about four hundred species exist to-day as a remnant of its for- mer disjilay, although the pines still form extensive forests. The group is so diversified in its structure that all forms can not be included in a single description. The common pine {Pinns), therefore, will be taken as a type, to show the general Gymnosperm character. ]^74 PLANT STRUCTURES 95. The plant body. — The great body of the plant, often forming a large tree, is the sporophyte ; in fact, the gametophytes are not visible to ordinary observation. It should be remembered that the sporophyte is distinctly a sexless generation, and that it develops no sex organs. This great sporophyte body is elaborately organized for nutritive work, with its roots, stems, and leaves. These organs are very complex in structure, being made up of various tissue systems that are organized for special kinds of work. The leaves are the most variable organs, being differentiated into three distinct kinds — (1) foliage leaves, (2) scales, and (3) sporophylls, 96. Sporophylls. — The sporophylls are leaves set apart to produce sporangia, and in the pine they are arranged in a strobilus, as in the Horsetails and Club-mosses. As the group is lieterosporous, however, there are two kinds of sporophylls and two kinds of strobili. One kind of strobilus is made up of megasporophylls bearing mega- sporangia ; the other is made up of microsporophylls bear- ing microsporangia. These strobili are often spoken of as the " flowers " of the pine, but if these are flowers, so are the strobili of Horsetails and Club-mosses. 97. Microsporophylls. — In the pines the strobilus com- posed of microsporophylls is comparatively small (Figs. 145, fZ, 164). Each sporophyll is like a scale leaf, is nar- rowed at the base, and upon the lower surface are borne two prominent sporangia, which of course are microspo- rangia, and contain microspores (Fig. 146). These structures of Seed-plants all received names before they were identified with the corresponding struc- tures of the lower groups. The microsporophyll was called a stamen, the microsporangia ;;o/'?^^w-.S'acs, and the microspores pollen grains, or simply pollen. These names are still very convenient to use in connection with the Spermatophytes, but it should be remembered that they are simply other names for structures found in the lower groups. Fig. 145. Finns Larido, sliowing ti]) of branch bearing noedlc-lcaves, scale-leaves, and cones (strobili): a. very young carpellate cones, at time of pollination, borne at tip of the young shoot upon which new leaves are appearing; b, carpellate cones one year old; c, carpellate cones two years old, the scales spreading and shedding the seeds; d, young shoot bearing a cluster of staminate cones. — Caldwell. 176 PLANT STRUCTURES The strobilus composed of microsporophylls may be called the sfaminate strobilus — that is, one comjDosed of stamens ; it is often called the staminate cone, " cone " being the English translation of the word "strobilus." Frequently the staminate cone is spoken of as the "male cone," as it was once supposed that the stamen is the Fig. 146. Stamirate cone (strobilus) of pine (Pmtis): A, section of cone, sliowing microsporophylls (stamens) bearing microsporangia; B, longitudinal section of a single stamen, showing the large sporangium beneath ; C, cross-section of a sta- men, showing the two sporangia; Z>, a single microspore (pollen grain) much en- larged, showing the two wings, and a male gametophyte of two cells, the lower and larger (wall cell) developing the pollen tube, the upper and smaller (genera- tive cell) giving rise to the sperms. — After Sthasburger. male organ. This name should, of course, be abandoned, as the stamen is now known to be a microsporophyll, which is an organ produced by the sjiorophyte, which never pro- duces sex organs. It should be borne distinctly in mind that the stamen is not a sex organ, for the literature of botany is full of this old assumption, and the beginner is in SPERMATOPIIYTKS: c;YM.\( JSl'HUMS ] 77 danger of becoming confused and of forgetting that pollen grains are asexual spores. 98. Megasporophylls. — The strobili composed of mega- sporophylls become much larger than the others, forming Fio. 147. Pimts sylvestrig, showing mature cone partly sectioned, and showing car- pels (sq, sq^, sq^) with seeds in their axils (q), in which the embryos (em) may be distinguished; A, a young carpel with two megaspcrangia; B, an old carpel with mature seeds (ch), the micropyle being below (Jf/").— After Besset. the well-known cones so characteristic of pines and their allies (Figs. US, a, b, c, 163). Each sporophyll is some- what leaf-like, and at its base upon the upper side are two megasporangia (Fig. 147). It is these sporangia which are peculiar in each producing and retaining a solitary large megaspore. This megasporc rct^embles a sac-like cavity in 178 PLANT STRUCTURES the body of the sporangium (Fig. 148, d), and was at first not recognized as being a spore. These structures had also received names before they were identified with the corresponding structures of the lower groups. The megasporophyll was called a carpel, the megasporangia ovules, and the megaspore an embryo- sac, because the young embryo was observed to develop Avithin it (Fig. 147, em). The strobilus of megasporophylls, therefore, may be called the carjjellate strohilus or carpellate cone. As the carpel enters into the organization of a structure known as the pistil, to be described later, the cone is often called the pistillate cone. As the staminate cone is sometimes wrongly called a "male cone," so the carpellate cone is wrongly called a ''female cone," the old idea being that the carpel with its ovules represented the female sex organ. The structure of the megaspo- rangium, or ovule, must be known. The main body is the nucelhis (Figs. 148, c,. 149, nc) ; this sends out from near its base an outer membrane [integwnent) which is distinct above (Figs. 148 i, 149 i), covering the main part of the nucellus and projecting beyond its apex as a prominent neck, the passage through which to the apex of the nucellus is called the micropyle ("little gate") (Fig. 148, a). Cen- trally placed within the body of the nucellus is the conspicuous cavity called the embryo-sac (Fig. 148, d), in reality the retained megaspore. The relations between integument, micropyle, nucellus, and embryo-sac should be kept clearly in mind. In the Fig. 148. Diagram of the carpel structures of pine, showing the heavy scale {A) which bears the ovule (B), in which are seen the micropyle (a), integument ib), nucellus (c), embryo sac or mega- spore ((?).— Moore. SPERMATOPHYTES : GYMNOSPEKMS 179 nc pine the micropyle is directed downward, toward the base of the sporophyll (Figs. 147, 148). 99. Female gametophyte. — The female gametophyte is always produced by the germination of a megaspore, and therefore it should be jjroduced by the so- called embryo-sac with- in the ovule. This im- bedded megaspore ger- minates, just as does the megaspore of Se- laginella or Isoetes, by cell division becoming filled with a compact mass of nutritive tissue representing the ordi- nary cells of the female prothallium (Fig. 149, e). This prothallium naturally does not protrude beyond the boundary of the mega- spore wall, being com- pletely surrounded by the tissues of the sporangium. It must be evident that this gametophyte is abso- lutely dependent upon the sporophyte for its nutrition, and remains not merely attached to it, but is actually im- bedded within its tis- sues like an in-ternal parasite. So conspicuous a tissue within the ovule, as well as in the seed into which the Fig. 149. Diagrammatic section through ovule (megasporangium) of spruce (Picea). showing integument (i), nucellus (nc), endosperm or female gametophyte (e) which fills the large megaspore imbedded in the nucellus, two archegonia (a) with short neck (c) and venter containing the egg (o). and position of ger- minating pollen grains or microspores (p) whose tubes (/) penetrate the nucellus tissue and reach the archegonia.— After Schimper. XgO PLANT STEUCTURES ovule develops, did not escape early attention, and it was called e)idosperm, meaning " within the seed." The endo- sperm of Gymnosperms, therefore, is the female gameto- phyte. At the margin of the endosperm nearest the micropyle regular flask-shaped archegonia are developed (Fig. 149, a), making it sure that the endosperm is a female gameto- phyte. It is evident that the necks of these archegonia (Fig. 149, c) are shut away from the approach of sperms by swimming, and that some new method of approach must be developed. 100. Male gametophyte. — The microspores are developed in the sporangium in the usual tetrad fashion, and are pro- duced and scattered in very great abundance. It will be remembered that the male gametophyte developed by the microspore of Selaginella is contained entirely within the spore, and consists of a single ordinary prothallial cell and one antheridium (see § 89). In the pine it is no bet- ter developed. One or two small cells appear, which may be regarded as representing prothallial cells, while the rest of the gametophyte seems to be a single antheridium (Fig. 146, D). At first this antheridium seems to consist of a large cell called the toall cell, and a small one called the generative cell. Sooner or later the generative cell divides and forms two small cells, one of Avhich divides again and forms two cells called male cells, which seem to represent the sperm mother cells of lower plants. The three active cells of the completed antheridium, therefore, are the wall cell, with a prominent nucleus, and two small male cells which are free in the large wall cell. These sperm mother cells (male cells) do not form sperms within them, as there is no water connection be- tween them and the archegonia, and a new method of transfer is provided. This is done by the wall cell, which develops a tube, known as the pollen-tuhe. Into this tube the male cells enter, and as it penetrates among the cells SPEEMATOPHYTES : GYMNOSPEEMS 181 which shut off the archegonia it carries the male cells along, aud so they are brought to the archegonia (Fig. 150). Fig. 150. Tip of pollen tube of pine, showing the two male cells (A, B), two nuclei ( C) which accompany them, and the numerous food granules {D) : the tip of the tube is just about to enter the neck of the archegonium.— Caldwell. Fig. 151. Pollen tube passing through the neck of an archegonium of spruce (Picea), and containing near its tip the two male nuclei, wiich are to be discharged into the egg whose cytoplasm the tube is just en- tering.—After Strasbdkger. 101. Fertilization. — Before fertilization can take place the pollen-grains (microspores) must be brought as near as possible to the female gametophyte with its archegonia. The spores are formed in very great abundance, are dry and powdery, and are scattered far and wide by the wind. In the pines and their allies the pollen-grains are winged (Fig. 146, />), so that they are well organized for wind dis- tribution. This transfer of pollen is called pollination, and those plants that use the wind as an agent of transfer are said to be anemopliilous, or ''wind-loving." The pollen must reach the ovule, and to insure this it must fall like rain. To aid in catching the falling pollen the scale-like carpels of the cone spread apart, the pollen grains slide down their sloping surfaces and collect in a 30 18^ PLANT STKUCTURES little drift at the bottom of each carpel, where the ovules are found (Fig. 147, A, B). The flaring lips of the micro- pyle roll inward and outward as they are dry or moist, and by this motion some of the pollen-grains are caught and pressed down upon the apex of the nucellus. In this position the pollen-tube develops, crowds its way among the cells of the nucellus, reaches the wall of the embryo-sac, and penetrating that, reaches the necks of the archegonia (Fig. 149, jo, t) ; crowding into them (Fig. 151), the tip of the tube opens, the male cells are /!5^r'fH:?^^^;A^- sn v-^m. Fig. 152. Fertilization in spruce (Picea): B is an egg, in the tip of which a pollen tube (p) has entered and has discharged into the cytoplasm a male nucleus (sn). which is to unite with the egg (female) nucleus (on); C, a later stage in which the two nuclei are uniting. — After Schimpbr. discharged, one male cell fuses with the egg (Fig. 152), and fertilization is accomplished, an oospore being formed in the venter of the archegonium. It will be noticed that the cell which acts as a male gamete is really the sperm mother cell, which does not organize a sperm in the absence of a water connection. This peculiar method of transferring the male cells by means of a special tube developed by the antheridium is SPEKMATOPHYTES : GYMNOSPEKMS 183 called siplionogamy, which means "sexual reproduction by means of a tube." So important is this character among Spermatophytes that some have proposed to call the group Siphonogams. 102. Development of the embryo.— The oospore when formed lies at the surface of the endosperm (female gameto- phyte) nearest to the micropyle. As the endosperm is to supply nourishment to the em- bryo, this position is not the most favorable. Therefore, as in Selaginella, the oospore first develops a suspensor, which in pine and its allies becomes very long and often tortuous (Fig. 153, A, s). At the tip of the suspensor the cell or cells (em- bryo cells) which are to develop the embryo are carried (Fig. 153, A, ka), and thus become deeply buried, about centrally placed, in the endosperm. Several suspensors may start from as many archegonia in the same ovule, and several embryos may begin to develop, but as a rule only one survives, and the solitary completed embryo (Fig. 153, B) lies centrally imbedded in the endosperm (Fig. 153^/). The development of more than one embryo in a megasporangium (ovule) is called polyembryony, a phenomenon natural to Gymnosperms with their several archegonia upon a single gametophyte. 103. The seed. — While the embryo is developing some important changes are taking place in the ovule outside of the endosperm. The most noteworthy is the develop- ment of a special tissue that forms a hard bouy covering, Fig. 153. Embryos of pine: .1, very young embryos (ka) at the tips of long and contorted sus- pensors (s)\ B, older embryo, showing attachment to suspen- sor (,«), the extensive root sheath (wh), root tip (u's), stem tip (r), and cotyledons (c).— After Strasburger. 184 PLANT STRUCTURES known as the seed coat, or testa (Fig. 153a). The devel- opment of this testa hermetically seals the structures with- in, further development and activity are checked, and the living cells pass into the resting condition. This pro- tected structure with its dormant cells is the seed. In a certain sense the seed is a transformed ovule (mega- sporangium), but this is true only as to its outer configura- FiG. 153a. Pine seed. Fig. 154. Pine seedlings, showing fee long hypocotyl and the nnmerous cotyledons, with the old seed case still attached.— After Atkinson. SPEEMATOPIIYTES: GYMNOSPERMS 185 tion. If the internal structures be considered it is much more. It is made uj) of structures belonging to three gen- erations, as follows : (1) The old sporophyte is represented by seed coat and nucellus, (2) the endosperm is a gameto- phyte, while (3) the embryo is a young sporophyte. It can hardly be said that the seed is simple in structure, or that any real conception of it can be obtained without approach- ing it by way of the lower groups. The organization of the seed checks the growth of the embryo, and this development within the seed is known as Fig. 155. A cycad, Rliowiiii; the palm-like habit, with much branched leaves and scaly Ptem. — Frimi '■ Plant Relations." the infra-seminal development. In this condition the em- bryo may continue for a very long time, and it is a ques- tion whether it is death or suspended animation. Is a seed alive ? is not an easy question to answer, for it may be kept in a dried-out condition for years, and then when placed in suitable conditions awaken and put forth a living plant. 5 — g s c -a C3 2 a,-! fc > %'-•., Fio. 178. End of embryo-sac of Silphium, showing double fer- tilization : sy, synergid, the other having been destroyed by the pollen-tube ; o, egg with coiled male cell (spi) lying against its nucleus ; e, endo- sperm cell, with large coiled male cell dyjj) 'yi"n against it. — After Land. is still under discussion, represent nutritive cells they disappear very soon times they become very Fig. 179. One end of the embryo-sac in wake-robin {Tnllimn), showing endosperm (shaded cells) in which a young embryo is imbedded. — After Atkinson. 208 PLANT STEDCTUKES active and even divide and form a considerable amount of tissue, which usually nourishes the embryo until endosperm tissue is developed, and then becomes disorganized ; or even invades the tissue of the nucellus. 114. Development of embryo. — While the endosperm is forming, the oospore has germinated and the sporophyte embryo is developing (Fig. 180). Usually a suspensor, more or less distinct, but never so prominent as in Gymnosperms, is formed ; at the end of it the embryo is developed (Fig. 181), which, when completed, is more or less surrounded by nourish- ing endosperm (Fig. 183). The two groups of Angio- sperms differ widely in the struc- ture of the embryo. In Mono- cotyledons the axis of the em- bryo develops the root-tip at one end and the " seed-leaf " (coty- ledon) at the other, the stem-tip arising from the side of the axis as a lateral member (Fig. 182). This relation of organs recalls the embryo of Isoetes (see § 90). Naturally there can be but one cotyledon under such circum- stances, and the group has been named Monocotyledons. In Dicotyledons the axis of the embryo develops the root-tip at one end and the stem- tip at the other, the cotyledons (usually two) appearing as a pair of opposite lateral members on either side of the stem-tip (Fig. 181). This recalls the relation of parts in the embryo of Sekiginella (see § 89). As the cotyledons are lateral members their number may vary. In Gymno- sperms, whose embryos are of this typej there are often Fig. 180. Curved embryo-sac of arrowhead (Sagittar'ia), show- ing in the upper right end a young embryo, in the other end the antipodal cells cut off by a partition, and scattered through the sac a few free en- dosperm cells. — After Scuaff- NER. SPEKMATOPIIYTES . ANGIOSPERMS 200 several cotyledons in a cycle (Fig. 154) ; and in Dicotyle- dons there may be one or several cotyledons ; but as a pair of opposite cotyledons is almost without exception in the group, it is named Dicotyledons. The axis of the embryo between the root-tip and the cotyledons is called the hypocotyl (Figs. 154, 193, 194), which Fig. 181. Development of embryo of shepherd's purse {Capsella), a Dicotyledon: beginning with /, the youngest stage, and following the sequence to VI, the old- est stage, V represents the suspensor, c the cotyledons, s the stem-tip, w the root, h the root-cap. Note the root-tip at one end of the axis and the stem-tip at the other between the cotyledons. — After Hanstein. means " under the cotyledon," a region which shows pecul- iar activity in connection with the escape of the embryo from the seed. Formerly it was called either caulicle or radicle. In Dicotyledons the stem-tip between the coty- 210 PLANT STRUCTURES ledons often organizes the rudiments of subsequent leaves, forming a little bud which is called the plmnule. Embryos differ much as to com- pleteness of their development within the seed. In some plants, especially those which are parasitic or sapro- phytic, the embryo is merely a small mass of cells, without any organiza- tion of root, stem, or leaf. In many cases the embryo becomes highly de- veloped, the endosperm being used up and the cotyledons stuffed with food material, the plumule contain- ing several well - organized young leaves, and the embryo completely filling the seed cavity. The com- mon bean is a good illustration of this last case, the whole seed within the integument consisting of the two large, fleshy cotyledons, between which lie the hypocotyl and a plu- mule of several leaves. 115. The seed. — As in Gymno- sperms, while the processes above described are taking place within the ovule, the tissue is developing that forms the hard seed-coat or testa (Fig. 183). When this hard coat is fully developed, the activities within cease, and the whole structure passes into that condition of suspended animation which is so little understood, and which may continue for a long time. The testa is variously developed in seeds, sometimes being smooth and glistening, sometimes pitted, sometimes rough with warts or ridges. Sometimes prominent append- ages are produced which assist in seed-dispersal, as the wings in Catcilim or Bignonia (Fig. 184), or the tufts of Fig. 182. Yoang embryo of water plantain (Alisma), a Monocotyledon, the root being organized at one end (next the snspensor), the single cotyledon (C) at the other, and the stem- tip arising from a lateral notch (»). — After Han- stein. SPERMATOPHYTES : ANGIOSPERMS 211 Fig. 183. The two figures to the left are seeds of violet, one showing the black, hard testa, the other being sectioned and showing testa, endosperm, and imbedded embryo; the figure to the right is a section of a pepper fruit (Piper), showing modified ovary wall (pc\. seed testa (sc), nucellus tissue (jj), endosperm {en), and embryo («m). — After Baillon. hair on the seeds of milkweed, cotton, or fireweed (Fig. 185). For a fuller account of the methods of seed-dispersal see Plant Relations, Chapter VI. Fig. 184. A winged seed of Bignonia.— After Strasburger. 116. The fruit— The effect of fertilization is felt beyond the boundaries of the ovule, which forms the seed. The ovary is also involved, and becomes more or less modiiied. It enlarges more or less, sometimes becoming remarkal)ly enlarged. It also changes in structure, often becoming hard or parchment-like. In case it contains several or numerous seeds, it is organized to open in some way and discharge them, as in the ordinary jjods and capstiles (Fig. 185). In case there is but one seed, the modified ovary 212 PLANT STKUCTUKES wall may invest it as closely as another integument, and a seed-like fruit is the result — a fruit which never opeus and is practically a seed. Such a fruit is known as an ahene, and is very characteristic of the greatest Angiosperm family, the Compositas, to which sunflowers, asters, golden- rods, daisies, thistles, dandelions, etc., belong. Dry fruits which do not open to discharge the seed often bear appendages to aid in dispersal by wind (Figs. 186, 187), or by animals (Fig. 188). Capsules, pods, and akenes are said to be dry fruits, but in many cases fruits ripen fleshy. In the peach, plum, cherry, and all ordinary '' stone fruits," the modified ovary wall or- ganizes two layers, the inner being very hard, forming the " stone," the outer being pulpy (Fig. 189), or vari- ously modified (Fig. 190). In the true berries, as the grape, currant, tomato, etc., the whole ovary becomes a thin-skinned pulpy mass in which the seeds are imbedded. In some cases the effect of ferti- lization in chang- ing structure is felt beyond the ovary. In the ap- W/jj '','.'(: pie, pear, quince, and sucR fruits, the pulpy part is the modified calyx (one of the Fiq. ISe. winged fruit of maple.— After Kerner. Fig. 185. A pod of fireweed {EjAlobi'im) opening and exposing its plumed seeds which are transported by the wind.— After Beal. SPEEMATOPHYTES : ANGIOSPERMS 213 floral leayes), tlie ovary and its contained seeds being repre- sented by the '^core." In other cases, the end of the stem bearing the ovaries (receptacle) becomes enlarged and pulpy, as in the strawberry (Fig. 191). This effect some- times involves even more than the ...^-^i/MMm^*.,^ parts of a single flower, a whole . fl o w e r - c 1 u s t e r, ^ with its axis ami bracts, becoming ,; an enlarged pulpy mass, as in the pineapple (Fig. - \->- 192). ■ ' The term "fruit," therefore, Fig. 187. A ripe dandelion head, showing the mass of plumes, a few seed-like fruits (akenes) with their plumes still attached to the receptacle, and two fallen off. — After Kernek. Fig. 188. An akene of beg- gar ticks, showing the two barbed appendages which lay hold of animals. — Af- ter Beal. 32 Fig. 189. To the left a section of a peach (fruit), showing pulp and stone formed from ovary wall, and the contained seed (kernels ; to the right the fruit of almond, which ripens dry.— After Gray. 2U PLANT STRUCT UKES is a very indefinite one, so far as the structures it includes are concerned. It is simply an effect which follows fer- tilization, and involves more or less of the structures adja- FiG. 190. Fruit of nutmeg (Myristica) : A, section of fruit, sliovving seed within the heavy wall ; B. section of seed, showing peculiar convoluted and hard endosperm {m) in which an embryo (;;) is imbedded. — After Berg and Schmidt. cent to the seeds, only to the ovary Fig. 191. Fruit of straw- berry, showing the per- sistent calyx, and the en- larged pulpy receptacle in which numerous sim- ple and dry fruits (a- kenesi are imbedded. — After Bailet. lodgment. If the devices for burial, As has been seen, this effect may extend wall, or it may include the calyx, or it may be specially directed toward the receptacle, or it may embrace a whole flower-cluster. It is what is called a physiological effect rather than a defi- nite morphological structure. 117. Germination of the seed. — It has been pointed out (§ 103) that the so-called " germination of the seed " is not true germination like that of spores. It is the awakening and es- cape of the young sporophyte, which has long before passed through its germination stage. By various devices seeds are separ rated from the parent plant, are dis- persed more or less widely, and find lodgment is suitable, there are many such as twisting stalks and awns, bur- SPERMATOPHYTES: ANGIOSPERMS 215 rowing animals, etc. The period of rest may be long or short, but sooner or later, under the influence of moisture, suitable temperature, and oxygen the quiescent seed begins to show signs of life. The sporophyte within begins to grow, and the seed coat is broken or penetrated through some thin spot or Pig. 102. Pineapple: .-1. the cluster of fruits forming the so-called "fruit"; i?, single flower, showing small calyx and more prominent corolla; C, section of flower, showing the floral organs arising above the ovary (ei)igynous). — .4, B after Koch; P after Lb Maout and Decaisne. opening. The root-tip emerges first, is protruded still farther by the rapid elongation of the hypocotyl, soon curves toward the earth, penetrates the soil, and sending out rootlets, becomes anchored. After anchorage in the 21C PLANT STRUCTURES soil, the hypocotyl again rapidly elongates and develops a strong arch, one of whose limbs is anchored, and the other is pulling npon the cotyledons (Fig. 193). This pull finally frees the cotyledons, the hypocotyl straightens, the cotyle- FiG. 193. Germination of the garden bean, showing the arch of the hypocotyl above ground, its pnll on the seed to extricate the cotyledons and plumule, and the final straightening of the stem and expansion of the young leaves.— After Atkinson. dons are spread out to the air and light, and the young sporophyte has become independent (Fig. 194). In the grain of corn and other cereals, so often used in the laboratory as typical Monocotyledons, but really excep- tional ones, the embryo escapes easily, as it is placed on one side of the seed near the surface. The hypocotyl and stem split the thin covering, and the much-modified cotyle- don is left within the grain to absorb nourishment. In some cases the cotyledons do not escape from the seed, either being distorted with food storage (oak, buck- eye, etc.), or being retained to absorb nourishment from the endosperm (palms, grasses, etc.). In such cases the stem-tip is liberated by the elongation of the petioles of the SPEEMATOl'IIYTES : ANGIOSPEEMS 217 cotyledons, and the seed coat containing the cotyledons remains like a lateral appendage upon the straightened axis. It is also to be observed in many cases that the young root system, after gripping the soil, contracts, drawing the young plant deeper into the ground. 118. Summary from Angio- sperms. — At the beginning of this chapter (§ 107) the characters of the Gymnosperms were summar- ized which distinguished them from Angiosperms, whose con- trasting characters may be stated as follows : (1) The microspore (pollen- grain), chiefly by insect pollina- tion, is brought into contact with the stigma, which is a receptive region on the surface of the car- pel, and there develops the pollen- tube, which penetrates the style to reach the ovary cavity which contains the ovules (megasporan- gia). The impossibility of con- tact betAveen pollen and ovule im- plies inclosed ovules and hence seeds, and therefore the name " Angiosperm." (2) The female gametophyte at the time of fertilization con- sists of only a few free nuclei and cells, usually seven in number. (3) The female gametophyte produces no archegonia, but a single naked egg. Fig. 194. Seedling of hornbeam ( CcD'pinus), showing primary root i^hiv) bearing rootlets (sw) upon which are numerous root hairs (;■), hypocotyl (h), cotyledons (c), young stem («), and first {1} and second W) true leaves.— After Sciitm- PER. CHAPTER XIII THE FLOWER 119. General characters. — In general the flower may be regarded as a modified branch of the sporophyte stem bear- ing sporophylls and usually floral leaves. Its representa- tive among the Pteridophytes and Gymnosperms is the stro- bilus, which has sporophylls but not floral leaves. Among Angiosperms it begins in a simple and somewhat indefinite way, gradually becomes more complex and modified, until it appears as an elaborate structure very eflicient for its purpose. This evolution of the flower has proceeded along many lines, and has resulted in endless diversity of structure. These diversities are largely used in the classification of Angiosperms, as it is supposed that near relatives are indi- cated by similar floral structures, as well as by other fea- tures. The signiflcance of these diversities is supposed to be connected with securing proper pollination, chiefly by insects, and favorable seed distribution. Although the evolution of flowers has proceeded along several lines simultaneously, now one feature and now another being emphasized, it will be clearer to trace some of the important lines separately. 120. Floral leaves. — In the simplest flowers floral leaves do not appear, and the flower is represented only by the sporophylls. Both kinds of sporophylls may be associated, in which case the flower is said to be 'perfect (Fig. 195) ; or they may not both occur in the same flower, in which case one flower is stmninate and the other jnstillate (Fig. 196). 218 THE FLOWER 219 When the floral leaves first ajjpear in connection with the sporophylls they are inconspicuous, scale-like bodies. In higher forms they become more prominent and inclose Fig. 195. Lizard's tail (Sawr^/rMs): A, tip of branch bearing leaves and elongated cluster of flowers; B, a single naked flower from A. showing sta- mens and four spreading and stigmatic styles; C, flower from another species, showing sub- tending bract, absence of floral leaves, seven stamens, and a syncarpous i)istil ; the flowers naked and perfect. -^Aftcr Engler. Fig. 196. Naked flowers of dif- ferent willows (Salix), each from the axil of a bract : a, b, c, staminate flowers ; d, *> fy pistillate flowers, the pistil composed of two car- pels (syncarpous). — After Wauming. Fig. 197. Flower of calamus (Acorvs). showing simple perianth, stamens, and syn- carpous pistil: a hypogynous flower without difl'erentiation of calyx and corolla. — After Engler. Fig. 199. Common flax (Linvm) : A. entire flower, showing calyx and corolla ; B, floral leaves re- moved, showing stamens and syi.carpous pistil ; C, a mature Fig. 198. Flowers of elm (Z7&»?m) : yl, branch capsule splitting open. —After bearing clusters of flowers and scaly buds ; Schimper. B, single flower, showing simple perianth and stamens, being a stamii ate flower ; C, flower showing perianth, stamens, and the two divergent styles stigmatic on inner surface, being a perfect flower; D. section through perfect flower, showing peri- anth, stamens, and pistil with two loculi each with a single ovule — After Englbr. Fig. 200. A flower of peony, showing the four sets of floral organs: Jc, the sepals, to- gether called the calyx; c, the petals, together called the corolla; a, the numerous stamens; g, the two carpels, which contain the ovules. — After Strasburger. THE FLOWER 221 the young sporophylls, but they are all alike, forming what is called the perianth (Figs. 197, 198). In still higher forms the perianth differentiates, the inner floral leaves become more delicate in texture, larger and generally brightly colored (Fig. 199, A). The outer set may remain scale-like, or become like small foliage leaves. When the dif- ferentiation of the peri- anth is distinct, the outer set of floral leaves is called the calyx, each leaf being a sejjcd; the inner set is the corolla, each leaf being a petal (Fig. 200). Sometimes, as in the lily, all the floral leaves become uniformly large and brightly colored, in which case the term perianth is retained (Fig. 201). In other cases, the calyx may be the large and colored set, but whenever there is a clear distinction between sets, the outer is the calyx, the inner the corolla. Both floral sets may not appear, and it has become the custom to ° o Fig. 201.— An caster -lily, a Monocotyledon, as the corolla, such showing perianth (a), stamens (6), stigma (O, flowers beinc called flower bud (d), and a carpel after the peri- ^ , anth has fallen (I), with its knob-like stigma, ap etaloUS, meaning long style, and slender ovary.— Caldwell. 222 PLANT STKUCTDRES " without petals." It is not always possible to tell whether a flower is apetalous — that is, whether it has lost a floral set which it once had — or is simply one whose perianth has not yet diflierentiatecl, in which case it would be a "primi- tive type." The line of evolution, therefore, extends from floweVs without floral leaves, or naked flowers, to those with a dis- tinctly differentiated calyx and corolla. 121. Spiral to cyclic flowers. — In the simplest flowers the sporophylls and floral leaves (if any) are distributed about an elongated axis in a spiral, like a succession of leaves. That part of the axis which bears the floral organs is for convenience called the receptacle (Fig. 202). As the recep- FiG. 202. A buttercup (Faminculus): a, complete flower, showing sepals, petals, sta- mens, and head of numerous carpels on a large receptacle; b, section showing relation of parts; a hypogynous, polypetalous, apocarpous, actinomorphic flower. —After Baiilon. tacle is elongated and capable of continued growth, an in- definite number of each floral organ may appear, especially of the sporophylls. With the spiral arrangement, there- fore, there is no definiteness in the number of floral organs ; there may be one or very many floral leaves, or stamens, or carpels. The spiral arrangement and indefinite numbers are features of the ordinary strobilus, and therefore such flowers are regarded as more primitive than the others. In higher forms the receptacle becomes shorter, the spiral more closely coiled, until finally the sets of organs THE FLOWER 223 appear to be thrown into rosettes or cycles. This change does not necessarily affect all the parts simultaneously. For example, in the common buttercup the sepals and petals are nearly in cycles, while the carpels are spirally arranged and indefinitely numerous on the head-like recep- tacle (Fig. 203). On the other hand, in the common water- FiG. 203. Flower of water-lily (Nymph, being a trimerous, pentacyclic flow- er.— After ScHiMPER. THE FLOWER 225 ring to the fact that the insertion of the other parts is under the ovary. Hypogyny is rerj largely displayed among flowers, but there is to be observed a tendency in some to carry the insertion of the outer parts higher up. When tlie outer parts arise from the rim of an urn-like outgrowth from the Fig. 205. Flowers of Rose family: i, a hypogynons flower of PotentUla, sepals, petals, and stamens arising from beneath the head of carpels; 5, a perigynous flower of Alcheinilla, sepals, petals, and stamens arising from rim of nrn-like pro- longation of the receptacle, which surrounds the carpel ; 3, an epigynons flower of the common apple, in which all the parts seem to arise from the top of the ovary, two of whose loculi are seen.— After Focke. receptacle, which surrounds the pistil or pistils, the flower is said to be perigynous (Figs. 205, ^, 206), meaning " around the pistil." Finally, the insertion is carried above the ovary, and sepals, petals, and stamens seem to arise from the toj? of the ovary (Fig. 205, 5), such a flower being epigynous, the outer parts appearing "upon the ovary." In such a case the ovary does not appear within the flower, but below it (Figs. 205, 252, 261), and the flower is often said to have an "inferior ovary." 123. Apocarpous to syncarpous flowers. — In the simpler flowers the carpels are entirely distinct, each carpel organ- 226 PLANT STRUCTURES izing a simple pistil, a single flower containing as many pistils as there are carpels, as in the buttercups (Figs. 200, 202). Such a flower is said to be apocarpous, meaning "carpels separate." There is a very strong tendency. FiQ. 206. Sweet-scented shrub (Calycanthus): A, tip of branch bearing flowers; B, section throiiijh flower, showing numerous floral leaves, stamens, and carpels, and also the develojiment of the receptacle about the carpels, making a perigynous flower. — After Thiebactlt. however, for the carpels of a flower to organize together and form a single compound pistil. In such a flower there may be several carpels, but they all appear as one organ (Figs. 195, C, 197, 198, D, 199, B), and the flower is said to be spicarpous, meaning "carpels together." 124. Polypetalous to s3anpetalous flowers. — The tendency for parts of the same set to coalesce is not confined to the carpels. Sepals often coalesce (Fig. 208), and sometimes stamens, but the coalescence of petals seems to be more important. Among the lower forms the petals are entirely separated (Figs. 199, A, 202, 203, 207), a condition which THE FLOWER 227 has received a variety of names, but probably the most common is 2)oly- petalous, meaning "petals many," although eleutheropetalous, meaning "petals free," is much more to the point. In the highest Angiosperms, how- ever, the petals are coalesced, form- ing a more or less tubular organ (Figs. 208-210). Such flowers are said to be sympetalous, meaning "petals united." The words gamo- petalous and monopetalous are also much used, but all three words refer to the same condition of the flower. Often the sympetalous corolla is difEerenti- FiG. 207. Flower of straw- berry, showing sepals, pet- als, numerous stamens, and head of carpels ; the flower is actinomorphic, hypogynons, and with no coalescence of parts.— Af- ter Bailey. Fig. 208. A flower of the tobacco plant: «, a complete flower, showing the calyx with its sepals blended below, the funnelform corolla made up of united petals, and the stamens jnst showing at the mouth of the corolla tube; b. a corolla tube split open and showing the five stamens attached to it near the base; c, a syncarpons pistil made up of two carpels, showing ovary, style, and stigma.— After Stbasbuboer. 228 PLANT STRUCTUEES ated into two regions (Fig. 210, h), a more or less tubular portion, the tuhe, and a more or less flaring portion, the limb. 125. Actinomorphic to zygomorpMc flow- ers.— In the simpler flowers all the mem- bers of any one cycle are alike ; the petals are all alike, the stamens are all alike, etc. Looking at the center of the flower, all the parts are re- peated about it like the parts of a radi- ate animal. Such a flower is actinomor- pJiic, meaning "ra- diate," and is often called a " regular flower." Although the term actinomor- phic strictly applies to all the floral organs, it is especially noteworthy in connection with the corolla, whose changes will be noted. Fig. 209. Flower of morning-glory (Jpomixa), with sympetalous corolla split open, showing the five attached stamens, and the superior ovary with prominent style and stigma ; the flower is hy- pogynous, sympetalous, and actinomorphic. — After Mkissner. Fig. 210. A group of sympetalous flower forms: a, a flower of harebell, showing a bell-shaped corolla; b, a flower of phlox, showing a tube and spreading limb; c, a flower of dead-nettle, showing a zygomorphic two-lipped corolla; d, a flower of toad-flax, showing a two-lipped corolla, and also a spur formed by the base of the corolla; e, a flower of the snapdragon, showing the two lips of the corolla closed. —After Gray. THE FLOWER 229 In many cases the petals are not all alike, and the radi- ate character, with its similar parts repeated about a cen- ter, is lost. In the common violet, for /^ - ^ example, one of the petals develops a spur (Fig. 211) ; in the sweet pea the petals are remarkably un- like, one being broad and erect, two small- er and drooping downward, and the other two much modi- fied to form together a boat-like structure which incloses the sporophylls. Such flowers are called zygomorpJiic, meaning "yoke-form," and they are often called " irregular flowers." When zygomorphic flowers are also sympetalous the corolla is often curiously shaped. A very common form Fig. 211. The pansy (Viola tricolor) : A, section showing sepals {I, I'), petals (c) one of which produces a spur (cs), the flower being zygomor- phic; B, mature fruit (a capsule) and persistent calyx (A;); C, the three boat-shaped valves of the fruit open, most of the seeds (s) having been discharged.— After Sachs. Fig. 212. Flower of a mint {Mentha aqnatica): A, the entire flower, showing calyx of united sepals, unequal petals, stamens, and style with two stigma lobes; B. a corolla split open, showing petals united and the four stamens attaclied to the tube; the flower is sympetalous and zygomorphic— After Werminq. 33 230 PLANT STKDCTUKES Fig. 213. Flower of a Labiate {Teucrlum), showing the calyx of coalesced sepals, the sympetalous and two-lipped (bilabi- ate) corolla with three petals (middle one largest) in the lower lip and two small ones in the upper, and the stamens and style emerging through a slit on the up- per side of the tube; a sympetalous and zygomorphic flower. — After Briquet. is the bilabiate, or "two-lipped," in which two of the petals usually organize to form one lip, and the other three form the other lip (Figs. 210, B^ c, d, e, 212, 213). The two lips may be nearly equal, the upper may stand high or overarch the lower, the lower may project more or less conspicuously, etc. 126. Inflorescence.— Very often flowers are soli- tary, either on the end of a stem or branch (Figs. 231, 236), or in the axil of a leaf (Fig. 258). But such cases grade insensibly into others Avhere a definite region of the plant is set aside to produce flowers (Figs. 253, 260). Such a region forms what is called the inflo- rescence. The various ways in which flowers are arranged in an inflorescence have received technical names, but they do not enter into our purpose here. They are simply dif- ferent ways in which plants seek to display their flowers so as to favor pollination and seed distribution. There are several tendencies, however, which may be noted. Some groups incline to loose clusters, either elon- gated (Fig. 260) or flat-topped (Fig. 253) ; others prefer large and often solitary flowers (Fig. 258) to a cluster of smaller ones ; but in the highest groups there is a distinct tendency to reduce the size of the flowers, increase their number, and mass them into a compact cluster. This ten- dency reaches its highest expression in tlie greatest family of the Angiosperms, the Compositse, of which the sunflower or dandelion can be taken as an illustration (Figs. 261, 262), in which numerous small flowers are closely packed together in a compact cluster which resembles a single large flower. It does not follow that all very compact inflorescences in- THE FLOWEK 231 dicate plants of high rank, for the cat-tail flag (Fig. 221) and many grasses have very compact inflorescences, and they are supposed to be plants of low rank. It is to be noted, however, that the very highest groups have settled upon this as the best type of inflorescence. 127. Summary. — In tracing the evolution of flowers, therefore, the following tendencies become evident : (1) from naked flowers to those with distinct calyx and corolla ; (2) from spiral arrangement and indefinite numbers to cyclic arrangement and definite numbers ; (3) from hypogynous to epigynous flowers ; (-t) from apocarpous to syncarpous pistils ; (5) from polypetalous to sympetalous corollas ; (6) from actinomorphic or regular to zygomorphic or irregular flowers ; (7) from loose to compact inflorescences. These various lines appear in all stages of advancement in different flowers, so that it would be impossible to deter- mine the relative rank in all cases. However, if a flower is naked, sjjiral, with indefinite numbers, hypogynous, and apocarpous, it would certainly rank very low. On the con- trary, the flowers of the Compositse have calyx and corolla, are cyclic, epigynous, syncarpous, sympetalous, often zygo- morphic, and are in a remarkably compact inflorescence, indicating the highest possible combination of characters. 128. Flowers and insects. — The adaptations between flowers and insects, by which the former secure pollination and the latter food, are endless. Many Angiosperm flowers, especially those of the lower groups, are still anemophilous, as are the Gymnosperms, but most of them, by the presence of color, odor, and nectar, indicate an adaptation to the visits of insects. This wonderful chapter in the history of plants will be found discussed, with illustrations, in Plant Relations, Chapter Yll. CHAPTEK XIV MONOCOTYLEDONS AND DICOTYLEDONS 129. Contrasting characters.— The two great groups of Angiosperms are quite distinct, and there is usually no dif- ficulty in recognizing them. The monocotyledons are usually regarded as the older and the simpler forms, and are represented by about twenty thousand species. The Dicotyledons are much more abundant and diversified, con- taining about eighty thousand species, and form the domi- nant vegetation almost everywhere. The chief contrasting characters may be stated as follows : Monocofi/Iedons. — (1) Embryo with terminal cotyledon and lat- eral stem-tip. This character is practically without exception. (2) Vascular bundles of stem scattered (Fig. 214). This means that there is no annual increase in the diameter of the woody stems, and no extensive branching, but to this there are some exceptions. (3) Leaf veins forming a closed system (Fig. 215, figure to left). As a rule there is an evident set of veins which run approximately parallel, and intricately branching between them is a system of minute veinlets not readily seen. The vein system does not end freely in the 232 Fig. 214. Section of stem of corn, showing the scattered bundles, indicated by black dots in cross-section, and by lines in longitudinal section. —From "Plant Relations." MONOCOTYLEDONS AND DICOTYLEDONS 233 margin of the leaf, but forms a '* closed venation," so that the loaves usually have an even {entire) margin. There are some notable exceptions to this character. (4) Cyclic flowers trim- crous. The "three-parted" Fm. 215. Two types of leaf venation: the figure to the left is from Solomon's seal, a Monocotyledon, and shows the principal veins parallel, the very minute cross veinlets being invisible to the naked eye; that to the right is from a willow, a Dicotyledon, and shows netted veins, the main central vein (midrib) sending out a series of parallel branches, which are connected with one another by a network of veinlets. — After Ettingshausen. flowers of cyclic Monocotyledons are quite characteristic, but there are some trimerous Dicotyledons. Dicotijledons. — (1) Embryo with lateral cotyledons and terminal stem-tip. (2) Vascular bundles of stem forming a hollow cylinder (Fig. 216, w). This means an annual increase in the diam- 234 FLAMT STKUCTUKES Fig. 216. Section across a young twig of box elder, showing the four stem regions: «, epidermis, represented by the heavy bounding line; c, cortex; w, vascular cyl- inder; }}, pith.— From "Plant Relations." eter of woody stems (Fig. 217, w), and a possible increase of the branch system and foliage dis- play each year. (3) Leaf veins form- ing an open system (Fig. 215, figure to right). The network of smaller veinlets between the larger veins is usually very evident, especially on the under surface of the leaf, suggesting the name "net- veined" leaves, in contrast to the '' parallel-veined " leaves of Mono- cotyledons. The vein system ends freely in the margin of the leaf, forming an "open venation." In consequence of this, although the leaf may remain entire, it very commonly be- comes toothed, lobed, and divided in various ways. Two main types of venation may be noted, which influence the form of leaves. In one case a single very prominent vein {rio) runs through the mid- dle of the blade, and is called the midrib. From this all the mi- nor veins arise as branches (Figs. 218, 219), and such a leaf Fia. 217. Section across a twig of box elder three years old, showing three annual rings, or growth rings, in the vascular cylinder; the radiating lines (m) which cross the vascular region (w) represent the pith rays, the princi- pal ones extending from the pith to the cor- tex (c).— From " Plant Relatione." MONOCOTYLEDONS AND DICOTYLEDONS 235 is said to be pinnate or pinnately veined, and inclines to elongated forms. In the other case several ribs of equal prominence enter the blade and diverge through it (Fig. 318). Such a leaf is palmate or palmately veined, and in- . clines to broad forms. (4) Cyclic flowers pentamerous or tetramerous. The flowers ''in fives" are greatly in the majority, but some Fig. 218. Leaves showing pinnate and palmate branching; the one to the left is from sumach, that to the right from buckeye. — Caldwell. very prominent families have flowers '' in fours." There are also dicotyledonous families with flowers "in threes/' and some with flowers " in twos." It should be remembered that no one of the above char- acters, unless it be the character of the embryo, should be depended upon absolutely to distinguish these two groups. 236 PLANT STRUCTURES It is the combination of characters which determines a group. Monocotyledons 130. Introductory. — This great group gives evidence of several distinct lines of development, distinguished by what may be called the working out of different ideas. In this way numerous families have resulted — that is, groups of Fig. 219. A leaf of honey locust, to show twice pinnate branching (bipinnate leaf ).— Caldwell. forms which seem to belong together on account of similar structures. This similarity of structure is taken to mean relationship. A family, therefore, is made up of a group of nearly related forms Opinions may differ as to what forms are so nearly related that they deserve to consti- tute a distinct family. A single family of some botanists may be " split up " into two or more families by others. Despite this diversity of opinion, most of the families are fairly well recognized. MONOCOTYLEDONS AND DICOTYLEDONS 237 Within a family there are smaller groups, indicating closer relationships, known as genera (singular, genus). For example, in the great family to which the asters belong, the different asters resemble one another more than they do any other members of the family, and hence are grouped together in a genus Aster. In the same family the golden- rods are grouped together in the genus Solidago. The different kinds of Aster or of Solidago are called species (singular also species). A group of related species, there- fore, forms a genus ; and a group of related genera forms a family. The technical name of a plant is the combination of its generic and specific names, the former always being written first. For example, Quercus alba is the name of the com- mon white oak, Quercus being the name of the genus to which all oaks belong, and alba the specific name which distinguishes this oak from other oaks. No other names are necessary, as no two genera of plants can bear the same name. In the Monocotyledons about forty families are recog- nized, containing numerous genera, and among these genera the twenty thousand species are distributed. It is evident that it will be impossible to consider such a vast array of forms, even the families being too numerous to mention. A few important families will be mentioned, which will serve to illustrate the group. 131. Pondweeds. — These are submerged aquatics, found in most fresh waters (some are marine), and are regarded as among the simplest Monocotyledons. They are slender, branching herbs, growing under water, but often having floating leaves, and sending the simple flowers or flower clusters above the surface for pollination and seed-distri- bution. The common pondweed (Potamogeton) contains numerous species (Fig. 320), while Kaias (naiads) and Zannichellia (horned pondweed) are common genera in ponds and slow waters. 238 TLAJs^T STRUCTURES The simple character of these forms is indicated by their aquatic habit and also by their flowers, which are mostly naked and with few sporophylls. A flower may consist of a single stamen, or a single carpel ; or there may be several stamens and carpels associated, but without any coalescence (Fig. 220, B). In the same general line with the pondweeds, but with more complex flowers, are the genera Sagittaria (arrow- PiG. 220. Pondweed {Potamogeton): A, branch with cUister (spike) of simple flowers, showing also the broad floating leaves and the narrow submerged ones; B, a sin- gle flower, showing the inconspicuous perianth lobes (c), the short stamens (a), and the two short styles with conspicuous stigmatic surfaces.— J^ after Reiohen- bach; B after Le Maout and Dbcaisnb. Fig. 221. Cat-tails ( Typha), ehowinp; the dense spikes of very simple flowers, each showing two regions, the lower the pistillate flowers, the upper the staminate. — From " Field, Forest, and Wayside Flowers." 240 PLANT STRUCTUEES leaf) and Alis7na (water-plantain), in which there is a dis- tinct calyx and corolla. The genus Typlia (cat-tail) is also an aquatic or marsh form of very simple type, the flow- ers being in dense cylindrical clusters (spikes), the upper flowers consisting of stamens, the lower of carpels, thus forming two very distinct re- gions of the spike (Fig. 221). 132. Grasses. — This is one of the largest and probably one of the most use- ful groups of plants, as well as one of the most peculiar. It is world-wide in its dis- tribution, and is re- markable in its dis- play of individuals, often growing so densely over large areas as to form a close turf. If the grass -like sedges be associated with them there are about six thousand species, representing nearly one third of the Mon- ocotyledons. Here belong the various cereals, sugar canes. Fig. 222. A common meadow graBS {Festuca) : A, portion of flower cluster (spikelet), showing the bracts, in the axils of two of which flowers are exposed ; B, a single flower with its envelop- ing bract, showing three stamens, and a pistil whose ovary bears two style branches with much branched stigmas.— After Strasburger. MONOCOTYLEDONS AND DICOTYLEDONS 241 bamboos, and pasture grasses, all of them immensely use- ful plants. The flowers are very simple, having no evident perianth (Fig. 323). Most commonly a flower consists of three sta- mens, surrounding a single carpel, whose ovary ripens into the grain, the characteristic seed-like fruit of the group. The stamens, however, may be of any number from one to six. The flowers, therefore, are naked, with indefinite num- bers, and hypogynous, indicating a comparatively simple type. It is also noteworthy that the group is aneniophilous. One of the noteworthy features of the group is the prominent development of peculiar leaves {bracts) in con- nection with the flowers. Each flower is completely pro- tected or even inclosed by one of these bracts, and as the bracts usually overlap one another the flowers are invisible until the bracts spread apart and permit the long dangling stamens to show themselves. These bracts form the so- called ''chaff" of wheat and other cereals, where they persist and more or less envelop the grain (ripened ovary). As they are usually called glumes, the grasses and sedges are said to be glumaceoiis plants. Grasses are not always lowly plants, for in the tropics the bamboos and canes form growths that may well be called forests. The grasses constitute the family GraminecB, and the sedges the family Cyperacem. 133. Palms. — More than one thousand species of palms are grouped in the family Palmacew. These are the tree Monocotyledons, and are very characteristic of the tropics, only the palmetto getting as far north as our Gulf States. The habit of body is like that of tree-ferns and Cycads, a tall unbranched columnar trunk bearing at its summit a crown of huge leaves which are pinnate or palmate in char- acter, and often splitting so as to appear lobed or comi)ound (Figs. 233, 324). The flower clusters are usually very large (Fig. 323), and each cluster at first is inclosed in a huge bract, which Fig. 2a3. A date palm, showing the unbranched columnar trunk covered with old leaf bases, and with a cluster of huge pinnate leaves at the top, only the lowest por- tions of which are shown ; two of the very heavy fruit clusters are also shown. — From " Plant Relations." MONOCOTYLEDONS AND DICOTYLEDONS 243 is often hard. Usually a perianth is present, but with no differentiation of calyx and corolla, and the flower parts are quite definitely in " threes," so that the cyclic arrangement with the characteristic Monocotyledon number appears. Fio. 234. A fan palm, with low stem mid . rown of large palmate leaves, which have split so as to appear palmately bruuched.— From •• Plant Relations." 134. Aroids. — This is a group of nearly one thousand species, most of them belonging to the family Aracecs. In our flora the Indian turnip or Jack-in-the-pulpit {AriscBma) (Fig. 225), sweetflag (Acorns), and skunk-cabbage {Symplo- carpus), may be taken as representatives ; while the culti- vated Calla-lily is perhaps even better known. The great display of aroids, however, is in the tropics, where they are endlessly modified in form and structure, and are erect, or climbing, or epiphytic. 244 PLANT STRUCTURES The flowers are usually very simple, often being naked, with two to nine stamens, and one to four carpels (Fig. Fig. 225. Jack-in-the-pulpit (Arismma). showing the overarching spathes; in one case a side view shows the naked tip of the projecting spadix.— After Atkinson. 197). They are inconspicuous and closely set upon the lower part of a fleshy axis, which is naked above and often MONOCOTYLEDONS AND DICOTYLEDONS 245 modified in a remarkable way into a club-shaped or tail-like often brightly colored affair. This singular flower-cluster with its fleshy axis is called a spaclix. The flowers often include but one sort of sporophyll, and staminate and pistillate flowers hold different positions upon the spadix (Fig. 226). The spadix is enveloped by a great bract, which sur- rounds and overarches like a large loose hood, and is called the spatlie. The spathe is exceedingly variable in form, and is often conspic- uously colored, forming in the Calla- lily the conspicuous white part, within which the spadix may be seen, near the base of which the flowers are found. In Jack-in-the-pulpit (Fig. 225) it is the overarching spathe which suggests the " pulpit." The spadix and spathe are the characteristic features of the group, and the spathe is variously modified in form, structure, and color for insect pollination, as is the peri- anth of other entomophilous groups. Aroids are further peculiar in hav- ing broad net-veined leaves of the Di- FiG. 226. Spadix of an Arum, with spathe re- moved, showing cluster of naked pistillate flow- ers at base, just above a cluster of staminate flowers, and the club- shaped tip of the spa- dix.—After WOSSIDLO. cotyledon type. Altogether they form a remarkably distinct group of Mon- ocotyledons. 135. Lilies. — The lily and its allies are usually regarded as the typical Monocotyledon forms. The perianth is fully developed, and is very conspicuous, either undifferen- tiated or with distinct calyx and corolla, and the flower is well organized for insect pollination. The flowers are either solitary or few in a cluster and correspondingly large, or in more compact clusters and smaller. In any event, the perianth is the conspicuous thing, rather than spathes or glumes. 34 246 PLANT STRUCTUEES In the general lily alliance, composed of eight or nine families, there are more than four thousand species, repre- senting about one fifth of all the Monocotyledons, and they are distributed everywhere. They are almost all terrestrial herbs, and are prominently geopMlous ("earth -lovers") — that is, they develop bulbs, rootstocks, etc., which enable them to disappear from above the surface during un- favorable conditions (cold or drought), and then to reappear rap- idly upon the return of favorable conditions (Figs. 227, 228, 231, 233). In the regular lily family {Liliacece) the flowers are hypogy- nous and actinomor- phic (Fig. 231), the six perianth parts are mostly alike and some- times sympetalous (as in the lily-of-the-val- ley, hyacinth, easter lily) (Figs. 201, 229), the stamens are usu- ally six (two sets), and the three carpels are syncarpous (Figs. 204, 230). This is a higher combination of floral characters than any of the preceding groups presents. Hypogyny and actinomorphy are low, but a conspicuous perianth, syn- carpy, and occasional sympetaly indicate considerable ad- vancement. Fig. 227. Wake-robin {Trillium), showing root- stock, from which two branches arise, each bear- ing a cycle (whorl) of three leaves and a single trimerons flower.— After Atkinson. MONOCOTYLEDONS AND DICOTYLEDONS 247 In the amaryllis family {Amarijllidacece), a higher fam- ily of the same general line, represented by species of Nar- cissus (jonquils, daffodils, etc.). Agave, etc., the flowers are distinctly epigynous. Fig 2^. Star-of -Bethlehem ( Ornithogalitm) : a, entire plant with tnberons base and tnmerous flowers; b. a single flower; c, portion of flower showing relation of parts, perianth lobes and stamens arising from beneath the prominent ovary (hy- pogynous); d, mature fruit; e. section of the syncarpous ovary, showing the three carpels and loculi.— After ScHisirER. In the iris family (Iridacece), the most highly specialized family of the lily line, and represented by the various spe- Fig. say. The Japan lily, showing a tubular puriaiuh, the parts of the perianth distinct above.— From " Field, Forest, and Wayside Flowers." MONOCOTYLEDONS AND DICOTYLEDONS 249 cies of Iris (flags) (Fig. 232), Crocus^ Gladiolus (Figs. 233, 234), etc., the flowers are not only epigynons, but some of them are zygomorphic. When a plant has reached both epigyny and zygomorphy in its flowers, it may be re- garded as of high rank. 136. Orchids.— In number of species this {Orchidacecs) is the greatest family among the Monocotyledons, the species being vari- ously estimated from six thousand to ten thousand, representing between one third and one half of all known Monocotyledons. In display of individuals, however, the orchids are not to be compared with the grasses, or even with lilies, for the various species are what are called "rare plants " — that is, not extensively distributed, and often very much restricted. Although there are some beautiful orchids in temperate regions, as species of Habenaria (rein- orchis) (Fig. 235), Pogonia^ Calopogoti, Ckdyjpso^ (Jypripe- dium (lady-slipper, or moccasin flower) (Fig. 236), etc., by far the greatest display and diversity are in the tropics, where many of them are brilliantly floAvered epiphytes (Fig. 237). Orchids are the most highly specialized of Monocoty- ledons, and their brilliant coloration and bizarre forms are associated with marvelous adaptation for insect visitation (see Plant Relations^ pp. 134, 135). The flowers are epigy- nous and strongly zygomorphic. One of the petals is re- markably modified, forming a conspicuous Up which is Fig. 230. Diagrammatic cross-section of ovary of Liliiim PhUadelphiciim, showing the three loculi, in each of which are two ovules (mega- sporangia); ,4, ovule; i>, integuments; C, nu- cellus ; D, embryo-sac (megaspore). — Cald- well. II |i III! l|l—WWHilliMIIII Ml PiQ. 231. The common dog-tooth violet, showing the large mottled leaves and con- spicuous flowers which are sent rapidly above the surface from the subterranean bulb (see cut w. the left lower corner), also some petals and stamens and the pistil dissected out.— From " Plant Relations." MONOCOTYLEDONS AND DICOTYLEDONS 251 modified in a great variety of ways, and a prominent, often very long, spur, in the bottom of which nectar is secreted, which must be reached by the proboscis of an insect (Fig. 235). The stamens are reduced to one or two, and welded with the style Fig. 233. Flower of flag {Iris), showing some of the sepals and petals, one of the three stamens, and the distinctly in- ferior ovary, being an epigy- nous flower. — After Gray. Fig. 234. Flower cluster of Gla- dlohis, showing somewhat zygo- morphic flowers.— Caldwell. Fig. 2.33. Gladiolus, showing tuberous subter- ranean stem from which roots descend, grass- like leaves, and somewhat zygomorphic flow- ers.—After Reichenbach. 252 PLANT STRUCTURES and stigmatic surface into an indistinguishable mass in the center of the flowers. The pollen-grains in each sac are sticky and cohere in a club-shaped mass {pollinium), which is pulled out and carried to another flower by the Fig. 235. A flower of an orchid (Habena- ria): at 1 the complete flower is shown, with three sepals behind and three pet- als iu front, the lowest one of which has developed a long strap-sha])ed portion (lip) and a still longer spur portion, the opening to which is seen at the base of the strap, and behind the spur the long inferior ovary (epigynous character) ; the two pollen sacs of the single stamen are seen in the center of the flower, di- verging downward, and between them stretches the stigma surface ; the rela- tion between pollen sacs and stigma sur- face is shown in -^ ; within each pollen sac is a mass of sticky pollen (pollini- um), ending below in a sticky disk, which may be seen in 1 and ;? ; in rf a pollen mass (a) is shown sticking to each eye of a moth.— After Geat. visiting insect. The whole structure indicates a very highly specialized type, elaborately organized for insect pollination. Another interesting epigynous and zygomorphic trop- ical group, but not so elaborate as the orchids, is repre- sented by the cannas and bananas (Fig. 120), common in cultivation as foliage plants, and the aromatic gingers. From the simple pondweeds to the complex orchids the evolution of the Monocotyledons has proceeded, and be- tween them many prominent and successful families have been worked out. Fig. 23C. A clump of lady-slippers (Cypripedium), showiiij,' the luibit of the plant and the general structure of the zygomorphic flower.— After Gibson. 254 PLANT STRUCTUKES Fig. SJ37. A group of orchids ( Cattleya), showing the very zygomorphic flowers, the lip being well shown in the flower to the left (lowest petal).— Caldwell. Dicotyledons 137. Introductory. — Dicotyledons form the greatest group of plants in rank and in numbers, being the most highly organized, and containing about eighty thousand species. They represent the dominant and successful vegetation in all regions, and are especially in the preponderance in tem- perate regions. They are herbs, shrubs, and trees, of every variety of size and habit, and the rich display of leaf forms is notably conspicuous. Two great groups of Dicotyledons are recognized, the ArcMchlaynydecB and the Sympetal,m. In the former there is either no perianth or its parts are separate (polypeta- lous) ; in the latter the corolla is sympetalous. The Archi- chlamydeae are the simpler forms, beginning in as simple a fashion as do the Monocotyledons ; while the Sympetalae MONOCOTYLEDONS AND DICOTYLEDONS 255 are evidently derived from them and become the most highly organized of all plants. The two groups each con- tain about forty thousand species, but the Archichlamydeas contain about one hundred and sixty families, and the Sympetalge about fifty. To present over two hundred families, containing about eighty thousand species, is clearly impossible, and a very few of the prominent ones will be selected for illustrations. ArcMcJilamydem 138. Poplars and their allies. — This great alliance repre- sents nearly five thousand species, and seems to form an isolated group. It is a notable tree assemblage, and appar- ently the most primitive and ancient group of Dicotyledons, containing the most important deciduous forest forms of r '^. -=SJ35:^*S««J£.. ^So topped clusters called --'f5,*?=i-A-£.-,T-«rj.?.=?^i'<:? nmbels (Figs. 252, A, 253). The branches of the clus- ter arise in cycles from the axis like the braces of an umbrella. As a re- sult of the close approxi- mation of the flowers the sepals are much reduced in size and often obsolete (Fig. 252, C). The Umbellifers are mainly perennial herbs of the north temperate re- gions, forming a very dis- tinct family, and contain- ing the following familiar forms : carrot {Daucus) (Fig. 252), parsnip {Pasti- naca), hemlock {Conium) (Fig. 253), pepper-and- salt {Erigenin), caraway {Carum), fennel {Fcenic- iihtm), coriander {Cori- andrum), celery {xipi- um), parsley {Petroseli- num), etc. Allied to the Umbellifers are the Ara- lias (Araliacece), and the Dogwoods {Cornacew). Fig. 253. Hemlock (Coniuni), an Umbellifer, showing the umbels, with the principal rays rising from a cycle of bracts (i/iro- lucre), and each bearing at its summit a secondary umbel with its cycle of second- ary bracts (involucel). — After Schimpeb. 268 PLANT STRUCTUKES SympetalcB 143. Introductory. — These are the highest and the most recent Dicotyledons. While they contain numerous shrubs and trees in the tropics, they are by no means such a shrub and tree group in the temperate regions as are the Archi- chlamydese. The flowers are constantly cyclic, the num- ber five or four is established, and the corolla is sympeta- lous, the stamens usually being borne upon its tube (Figs. 208, 209, 212). There are two well-defined groups of Sympetalae, distin- guished from one another by the number of cycles and the number of carpels in the flower. The group containing the lower forms is pentacyclic, meaning " cycles five," there being two sets of stamens. In it also there are five carpels, the floral formula being, Sepals 5, Petals 5, Stamens 5 + 5, Carpels 5. As the carpels are the same in number as the other parts, the flowers are called isocarpic, meaning " car- pels same." The group is named either Pentacyclm or Iso- carpce, and contains about ten families and 4,000 species. The higher groups, containing about forty families and 36,000 species, is tetracijclic, meaning " cycles four," and anisocarpic, meaning *' carpels not the same," the floral formula being. Sepals 5, Petals 5, Stamens 5, Carpels 2. The group name, therefore, is Tetracydm or Anisocarpoi. 144. Heaths. — The Heath family (Ericacece) and its allies represent about two thousand species. They are mostly shrubs, sometimes trailing, and are displayed chiefly in temperate and arctic or alpine regions, in cold and damp or dry places, often being prominent vegetation in bogs and heaths, to which latter they give name (Fig. 254). The flowers are pentacyclic and isocarpic, as well as mostly hyp- ogynous and actinomorphic. It is interesting to note that some forms are not sympetalous, the petals being distinct, showing a close relationship to the Archichlamydese. One of the marked characteristics of the group is the dehiscence MONOCOTYLEDONS AND DICOTYLEDONS 269 of the pollen-sacs by terminal pores, which are often pro- longed into tubes (Fig. 255). Fig. 254. Characteristic heath plants: ,4, B, (U Lyonia. showing sympetalous flowers and single style from the lobed syncarpons ovary; Z>, two forms of Casnojie, showing trailing habit, small overlapping leaves, and sympetalous flowers, but in the smaller form the petals are almost distinct.— After Drudb. Common representatives of the family are as follows : huckleberry (Gaijlussacia), cranberry and blueberry ( Vac- cinium), bearberry (Arcfostaphylos), trailing arbutus [Upi- 270 PLANT STRUCTURES gcea), wintergreen (GauUheria), heather {Calluna), moun- tain laurel {Kalmia), Azalea, Rhododefidron (Fig. 250), Indian pipe {Monotropa), etc. Fig. 255. Flowers of heath plants (Erica), showing compktc flowers (4), the sta- mens wlih "two-horned" anthers which discharge pollen through terminal pores, and the lobed syncarpous ovary with single style and prominent terminal stigma (B, C, i>).— After Drude. 145. Convolvulus forms. — The well-known morning-glory {Ipomcea) (Fig. 209) may be taken as a type of the Convol- MONUCOTYLEDONS AND DICOT i'LEDONS 271 vulus family {Cunvohmlacece). Allied with it are Poleino- nium and Phlox (Fig. 210, b) {Polemoniacece), the gentians (Gentianacece), and the dog-banes {Apocijnacem) (Fig. 257). It is here that the regular sympetalous flower reaches its highest expression in the form of conspicuous tubes, f un- FiG. 256. A cluster of Rhododendron flowers.— After IIooker. nels (Fig. 258), trumpets, etc. The flowers are tetracyclic and anisocarpic, besides being hypogynous and actinomor- phic. These regular tubular forms represent about five thousand species, and contain many of the best-known flowers. 272 PLANT STRUCTURES 146. Labiates. — This great family (LaMatce) and its alli- ances represent more than ten thousand species. The con- spicuous feature is the zygomorjihic flower, dif- fering in this regard from the Convolvulus forms, which they resemble in being tetracyclic and ani- socarjiic, as well as hypogy- nous. The irregularity consists in organizing the mouth of the sympetalous corolla into two "lips," resulting in the labiate or Fig. 2b7. A comuiou du^hauc X^-ipoci/nuin). —From "Field, Forest, aud Wayside Flowers." Fig. 258. The hedge bindweed { Convulcuius). showing the twining habit and the con- spicuous funnelform corollas. — From " Field, Forest, and Wayside Flowers." 274 PLANT STKUCTUEES MlaUate structure (Fig. 210, c, d, e), and suggesting the name of the dominant family. The upper lip usually con- tains two petals, and the lower three ; the two lips are some- times widely separated, and sometimes in close contact, and differ widely in relative prominence. Associated Avith zygomorphy in this group is a frequent reduction in the number of stamens, which are often four (Fig. 212) or two. The whole structure is highly special- ized for the visits of insects, and this great zygomorphic alliance holds the same relative position among Sympetalae as is held by the zygomorphic Le- gumes among Archi- chlamydeae. In the mint family, as the Labiates are often called, there are about two thousand seven hun- dred species, including mint {Mentha) (Fig. 213), dittany {Cunila), hyssop {Hyssopus), mar- joram {Origanu m ) , Fig. 259. Flowers of dead nettle (Xrt- miiim) : A, entire bilabiate flower ; B, section of flower, showing rela- tion of parts.— After Warming. Fig. 260. A labiate plant (Teucrium). show- ing branch with flower clusters (.-1), and side view of a few flowers {B), showing their bilabiate character.— After Briquet. MONOCOTYLEDONS AND DICOTYLEDONS 275 thyme {Thymus), balm {Melissa), sage {Salvia), catnip {Nepeta), skullcap {Sctitellaria) , horehound {Marruhium), iSvender {Lavandula), rosemary {Rosmarinus), dead nettle {Lamium) (Fig. 259), Teucriu?n (Figs. 213, 260), etc., a remarkable series of aromatic forms. Allied is the Nightshade family {Solanacew), with fif- teen hundred species, containing such common forms as the nightshades and potato {Solanu/n), tomato {Lycoper- sicum), tobacco {Nicotiana) (Fig. 208), etc., in which the corolla is actinomorphic or nearly so ; also the great Fig- wort family {ScrophulariacecB) , with two thousand species, represented by mullein ( Verhascum), snapdragon {Antir- rhinum) (Fig. 210, e), toad-flax {Linaria) (Fig. 210, c?), Pentstemo7i, speedwell ( Veronica), Gerardia, painted cup {Castilleia), etc.; also the Verbena family {Verbenacew), with over seven hundred species ; and the two hundred plantains {Planfaginacece), etc. 147. Composites. — This greatest and ranking family ( Co7nposit(e) of Angiosperms is estimated to contain at least twelve thousand species, containing more than one seventh of all known Dicotyledons and more than one tenth of all Seed-plants. Not only is it the greatest family, but it is the youngest. Composites are distributed everywhere, but are most numerous in temperate regions, and are mostly herbs. The name of the family suggests the most conspicuous feature — namely, the remarkably complete organization of the numerous small flowers into a compact head which resembles a single flower, formerly called a "compound flower." Taking the head of an Arnica as a type (Fig. 261), the outermost set of organs consists of more or less leaf-like bracts or scales {involucre), which resemble sepals ; within these is a circle of flowers with conspicuous yellow corollas {rays), which are zygomorphic, being split above the tubular base and flattened into a strap-shaped body, and much resembling petals (Fig. 261, A, D) ; within the Fig. 261. Flowers of Arnica: A, lower part of .stem, and upper part bearing a head, in which are seen the conspicuous rays and the disk; D, single ray flower, showing the corolla, tubular at base and strap-shaped above, the two-parted style, the tuft of pappus hairs, and the inferior ovary which develops into a seed-like fruit (akene); E, single disk flower, showing tubular corolla with spreading limb, the two-parted style emerging from the top of the stamen tube, the prominent pappus, and the inferior ovary or akene; C, a single stamen.— After Hoffman. 276 MONOCOTYLEDONS AND DICOTYLEDONS 277 ray-flowers is the broad expanse supplied by a very much broadened axis, and known as the dish (Fig. 261, A), which is closely packed with very numerous small and regular tubular flowers, known as disk-jioivers (Fig. 261, e). Fig. 262. The common dandelion (T'araa'aCTim): i, two flower stalks; in one the head is closed, showing the double involucre, the inner erect, the outer reflexed, in the other the head open, showing that all the flowers are strap-shaped; 2, a single flower showing inferior ovarj'. pappus, corolla, stamen tube, and two-parted style; 3, a mature akene; U, a head from which all but one of the akenes have been re- moved, showing the pitted receptacle and the prominent pappus beak.— After Strasburger. The division of labor among the flowers of a single head is plainly marked, and sometimes it becomes quite com- plex. The closely packed flowers have resulted in modify- ing the sepals extremely. Sometimes they disappear en- 3G 278 PLANT STRUCTURES tirely ; sometimes t^iey become a tuft of delicate hairs, as in Arnica (Fig. 261, D, E), thistle {Cnictis), and dandelion {Taraxacum) (Fig. 263), surmounting the seed-like akene and aiding in its transportation through the air ; sometimes they are converted into two or more tooth-like and often Fig. 263. Flowers of dandelion, showing action of style in removini; jxillen from the stamen tube: ;, style having elongated through the tube and carrying pollen; 2, style branches beginning to recurve; 3, style branches completely recurved.^ From " Field, Forest, and Wayside Flowers." barbed processes arising from the akene, as in tickseed (Coreopsis) and beggar-ticks (Fig. 188) or Spanish needles (Bidens), to lay hold of passing animals ; sometimes they become beautifully plumose bristles, as in the blazing star (Liatris) ; sometimes they simply form a more or less con- spicuous cup or set of scales crowning the akene. In all of these modifications the calyx is called pappus. The stamens within the corolla are organized into a tube by their coalescent anthers (Fig. 263), and discharge their pollen within, which is carried to the surface of the MONOCOTYLEDONS AND DICOTYLEDONS 270 head and exposed by the swab-like rising of the style (Fig. 263). The head is thus smeared with pollen, and visiting insects can not fail to distribute it over the head or carry it to some other head. In the dandelion and its allies the flowers of the disk are like the ray-flowers, the corolla being zygomorphic and strap-shaped (Figs. 263, 363). The combination of characters is sympetalous, tetracyc- lic, and anisocarpic flowers, which are epigynous and often zygomorphic, with stamens organized into a tube and calyx modified into a pappus, and numerous flowers organized into a compact involucrate head in which there is more or less division of labor. There is no group of plants that shows such high organization, and the Compositae seem to deserve the distinction of the highest family of the plant kingdom. The well-known forms are too numerous to mention, but among them, in addition to those already mentioned, there are iron-weed ( Verno7iia), Aster, daisy {Bellis), goldenrod {Soli dago), rosin-weed and compass-plant [Silph- ium), sunflower {Helianthus), Chrysatithefnum., ragweed {Ambrosia), cocklebur {Xanfhiu?n) , ox-eye daisy {Leucaii- themum), tansy {Tanaceium), wormwood and sage-brush {Artemisia), lettuce {Laduca), etc. CHAPTEE XV DIFFERENTIATION OF TISSUES 148. Introductory. — Among the simplest Tliallophytes the cells forming the body are practically all alike, both as to form and work. What one cell does all do, and there is very little dependence of cells upon one another. As plant bodies become larger this condition of things can not continue, as all of the cells can not be put into the same relations. In such a body certain cells can be related to the external food supply only through other cells, and the body becomes differentiated. In fact, the relating of cells to one another and to the external food-supply makes large bodies possible. The first differentiation of the plant body is that which separates nutritive cells from reproductive cells, and this is accomplished quite completely among the Thallophytes. The differentiation of the tissues of the nutritive body, however, is that which specially concerns us in this chapter. A tissue is an aggregation of similar cells doing similar work. Among the Thallophytes the nutritive body is prac- tically one tissue, although in some of the larger Thallo- phytes the outer and the inner cells differ somewhat. This primitive tissue, composed of cells with thin walls and ac- tive protoplasm, and to be regarded as the parent tissue, is called parencliyma. Among the Bryophytes, in the leafy gametophore and in the sporogonium, there is often developed considerable dissimilarity among the cells forming the nutritive body, but the cells may all still be regarded as parenchyma. It 280 DIFFERENTIATION OF TISSUES 281 is in the sporophyte of the Pteridophytes and Spermato- phytes that this differentiation of tissues becomes extreme, and tissues are organized which differ decidedly from parenchyma. This differentiation means division of labor, and the more highly organized the body the more tissues there are. All the other tissues are derived from parenchyma, and as the work of nutrition and of reproduction is ahvays retained by the parenchyma cells, the derived tissues are for mechanical rather than for vital purposes. There is a long list of these derived and me- chanical t' ^ues, some of them being of general occurrence, and others more restricted, and there is every gradation between them and the parenchyma from which they have come. We shall note only a few which are distinctly differentiated and which are common to all vascular plants. 149. Parenchyma. — The parenchyma of the vascular plants is typically made up of cells which have thin walls and whose three dimensions are approximately equal (Figs. 26-1, 265), though sometimes they are elongated. Until abandoned, such cells contain very active protoplasm, and it is in them that nutritive work and cell division are carried on. So long as these cells retain the power of cell division the tissue is called meristem, or it is said to be mcristematic, from a Greek word meaning "to divide."' When the cells stop dividing, the tissue is said to be permanent. The growing points of organs, as stems, roots, and leaves, are composed of parenchyma which is mcristematic (Figs. 266, 274), and meristem occurs wherever growth is going on. Pig. 264. Parenchyma and sclerenchyma from the stem of Fteris, in cross-section.— Cham- berlain. 282 PLANT STKUCTUEES 150. Mestome and stereome. — When the plant body be- comes complex a conductive system is necessary, so that the different regions of the body may be put into communi- cation. The material absorbed by the roots must be carried to the leaves, and the food manu- factured in the leaves must be carried to regions of growth and storage. This' business of transportation is provided for by the specially organized ves- sels referred to in preceding chapters, and all conducting tis- sue, of whatever kind, is spoken of collectively as mestorne. If a complex body is to main- tain its form, and especially if it is to stand upright and be- come large, it must develop structures rigid enough to fur- nish mechanical support. All the tissues which serve this pur- pose are collectively known as stereome. The sporophyte body of Pteridophytes and Spermato- phytes, therefore, is mostly made up of living and working parenchyma, which is traversed by mechanical mestome and stereome. 151. Dicotyl and Conifer stems. — The stems of these two groups are so nearly alike in general plan that they may be considered together. In fact, the resemblances were once thouglit to be so important that these two groups were put together and kept distinct from Monocotyledons ; but this was before the gametophyte structures were known to bear very different testimony. \ Fig. 265. Same tissues as in pre- ceding figure, in longitudinal sec- tion, the parenchyma showing nuclei.— Chamberlain. DIFFEKENTIATION OF TISSUES 283 At the apex of the growing stem there is a group of active meristem cells, from which all the tissues are de- rived (Fig. 266). This group is known as the apical group. Below the apical group the tissues and regions of the stem begin to appear, and still farther down they become dis- tinctly differentiated, passing into permanent tissue, the apical group by its divisions continually adding to them and increasing the stem in length. Just behind the apical group, the cells begin to give the appearance of being organized into three great embryonic re- gions, the cells still remaining meristem- atic (Fig. 266). At the surface there is a single layer of cells distinct from those within, known as the dermatogen^ or " skin-producer," as farther down, where it becomes permanent tissue, it is the epidermis. In the center of ^the embryonic region there is organized a solid cylinder of cells, distinct from those around it, and called the plerome, meaning "that which fills up." Farther down, where the plerome passes into permanent tissue, it is called the central cylinder or stele ("column"). Between the plerome and dermatogen is a tissue region called the perihlem, meaning "that which is put around," and when it becomes permanent tissue it is called the cortex, meaning "bark " or "rind," Putting these facts together, the general statement is that at the apex there is the apical group of meristem cells ; Fig. 266. Section through growing point of stem of Hippuris : below the growing point, composed of a uniform meristem tissue, the three embry- onic regions are outlined, showing the dermato- gen (d, d), the central plerome (p.p). and be- tween them the periblem. — After De Bart. 284 PLANT STEUCTUKES below them are the three embryonic regions, dermatogen, periblem, and plerome ; and farther below these three regions pass into permanent tissue, organizing the epider- mis, cortex, and stele. The three embryonic regions are usually not so distinct in the Conifer stem as in the Dico- tyl stem, but both stems have epidermis, cortex, and stele. Epidermis. — The epidermis is a protective layer, whose cells do not become so much modified but that they may be regarded as parenchyma. It gives rise also to super- ficial parts, as hairs, etc. In the case of trees, the epidermis does not usually keep up with the increasing diameter, and disappears. This puts the work of protection upon the cortex, which organizes a superficial tissue called cork, a prominent part of the structure known as hai'lc. Cortex. — The cortex is characterized by containing much active parenchyma, or primitive tissue, being the chief seat of the life activities of the stem. Its superficial cells, at least, contain chlorophyll and do chlorophyll work, while its deeper cells are usually temporary storage places for food. The cortex is also char- acterized by the development of stereome, or rigid tissues for me- chanical support. The stereome may brace the epidermis, forming the hypodermis ; or it may form bands and strands within the cor- tex ; in fact, its amount and ar- rangement differ widely in differ- ent plants. The two principal stereome tis- sues are collenchynia and scleren- chyma, meaning "sheath-tissue" and " hard-tissue " respectively. In collenchyma the cells are thick- ened at the angles and have very elastic walls (Fig. 267), making the tissue well adapted for parts which are growing Fig. 267. Some collenchyma cells from the stem of a com- mon dock iRuinex), showing the cells thickened at the angles.— Chamberlain. DIFFERENTIATION OF TISSUES 285 in length. The chief mechanical tissue for parts which have stopped growing in length is sclerenchyma (Figs. 264, 265). The cells are thick-walled, and usually elongated and with tapering ends, including the so-called "fihers." Fig. 268. Sections through an open collateral vascular bundle from a sunflower stem; A, cross-section; B. longittulinal section: the letters in both referring to the same structures; 3/, pith; X xylom. containing spiral {c, .«') and pitted (t. I') vessels; C, cambium; P, phloem, containing sieve vessels (sb)\ b, a mass of bast fibers or sclerenchyma; ic, pith rays between the bundles; e, the bundle sheath; R, cor- tex.—After Vines. Stele. — The characteristic feature of the stele or central cylinder is the development of the mestome or vascular 286 PLANT STRUCTURES tissues, of which there are two prominent kinds. The tracheary vessels are for water conduction, and are cells with heavy walls and usually large diameter (Fig. 268). The thickening of the walls is not uniform, giving them a very characteristic appearance, the thickening taking the form of spiral bands, rings, or reticulations (Fig. 308, B). Often the reticulation has such close meshes that the cell wall has the appearance of being covered with thin spots, and such cells are called " pitted vessels." The vessels with spirals and rings are usually much smaller in diameter than the pitted ones. The true tracheary cells are more or less elongated and without tapering ends, fitting end to end and forming a continuous longitudinal series, suggesting a trachea, and hence the name. In the Conifers there are no true tracheary cells, as in the Dicotyledons, except a few small spiral vessels which are formed at first in the young stele, but the tracheary tissue is made up of traclieids, mean- ing " trachea - like," differing from trachem or true tracheary vessels in having tapering ends and in not forming a continu- ous series (Fig. 2(50). The walls of these tracheids are "pitted" in a way which is characteristic of Gymnosperms, the "pits" appearing as two concentric rings, called "bordered pits." The other prominent mes- tome tissue developed in the stele is the sieve vessels, for the conduction of organized food, chiefly proteids (Fig. 2G8). Sieve cells are so named because in their walls special areas are organized which are perforated like the lid of a pepper- FiG. 269. Tracheids from wood of pine, showing tapering ends and bordered pits.— Chamberlain. DIFFERENTIATION OF TISSUES 287 box or a "sieve." These perforated areas are the sieve- plates, and through them the vessels communicate with one another and with the adjacent tissue. The tracheary and sieve vessels occur in separate strands, the tracheary strand being called xylem (" wood "), the sieve strand phloem. (" bark "). A xylem and a phloem strand are usually organized together to form a vascular hiindle, and it is these fiber-like bundles which are found traversing the stems of all vascular plants and appearing conspicuously as the veins of leaves. Among the Dicotyls and Conifers the vascular bundles appear in the stele in such a way as to outline a hollow cylinder (Fig. 216), the xylem of each bundle being toward the center, the phloem toward the circumference of the stem. The undifEerenti- ated parenchyma of the stele which the vascular cylinder incloses is called the pith. In older parts of the stem the pith is often abandoned by the activities of the plant, and either remains as a dead spongy tissue, or disappears en- tirely, leaving a hollow stem. Between the bundles form- ing the vascular cylinder there is also undifferentiated parenchyma, and as it seems to extend from the pith out between the bundles like "rays from the sun," the rays are called pitli rays. Such vascular bundles as described above, in which the xylem and phloem strands are " side-by-side " upon the same radius, are called collateral (Fig. 270). One of the pecul- iarities of the collateral bundles of Dicotyls and Conifers, however, is that when the two strands of each bundle are organized some meristem is left between them. This means that between the strands the work of forming new cells can go on. Such bundles are said to be open ; and the open collateral bundle is characteristic of the stems of the Dico- tyls and Conifers. The meristem between the xylem and phloem of the open bundle is called cambium (Figs. 268, 270). The cam- bium also extends across the pith rays between the bundles. 288 PLANT STRUCTURES connecting the cambium in the bundles, and thus forming a cambium cylinder, which separates the xylem and phloem of the vascular cylinder. This cambium continues the f or- FiG. 270. Cross-section of open collateral vascular bundle from stem of castor-oi! plant (Rlclnus), showing pith cells (m), xylem containing spiral {t) and pitted {g) vessels, cambium of bundle (c) and of pith rays (cb), phloem containing sieve ves sels (y\ three bundles of bast fibers or sclerenchyma (b), the bundle sheath con' taining starch grains, and outside of it parenchyma of the cortex (»•).— After Sachs mation of xylem tissue on the one side and phloem tissue on the other in the bundles, and new parenchyma between the bundles, and so the stem increases in diameter. If the stem lives from year to year the addition made by the cam- bium each season is marked off from that of the previous season, giving rise to the so-called growth rings or annual rings, so conspicuous a feature of the cross-section of tree DIFFEKENTIATION OF TISSUES 289 trunks (Fig. 217). This continuous addition to the vessels increases the capacity of the stem for conduction, and per- mits the furtlier extension of branches and a larger display of leaves. The annual additions to the xylem are added to the in- creasing mass of wood. The older portions of the xylem mass are gradually abandoned by the ascending water C'sap"), often change in color, and form the lieart-ivood. The younger portion, through which the sap is moving, is the sap-wood. It is evident, however, that the annual ad- ditions to the phloem are not in a position for permanency. The new phloem is deposited inside of the old, and this, to- gether with the new xylem, presses upon the old phloem, which becomes ruptured in various ways, and rapidly or very gradually peels off, being constantly renewed from within. It is the protecting layers of cork (see this section under Cortex), the old phloem, and the new phloem down to the cambium, which constitute the so-called bark of trees, a structure exceedingly complex and extremely vari- able in different trees. The stele also frequently develops stereome tissue in the form of sclerenchyma. These thick-walled fibers are often closely associated with one or both of the vascular strands of the bundles (Fig. 270), and lead to the old name Jibro- vascular bundles. To sum up, the stems of Dicotyledons and Conifers are characterized by the development of a vascular cylinder, in which the bundles are collateral and open, permitting increase in diameter, extension of the branch system, and a continuous increase in leaf display. 152. Monocotyl steins. — In the stems of Monocotyledons there is the same apical development and differentiation (Fig. 266). The characteristic difference from the Dicotyl and Conifer type, just described, is in connection with the development of the vascular bundles in the stele. Instead of outlining a hollow cylinder, the bundles are scattered 290 PLANT STRUCTURES through the stele (Fig. 214). This lack of regularity would interfere with the organization of a cambium cylinder, and we find the bundles collateral but closed — that is, with no meristem left between the xylem and phloem (Fig. 371). Fig. 271. Cross-section of a closed collateral bundle from the stem of corn, showing the xylem with annular (r), spiral (s), and pitted {g^ vessels; the phloem contain- ing sieve vessels (v), and separated from the xylem by no intervening cambium; both xylem and phloem surrounded by a mass of sclerenchyma (fibers); and in- vesting vessels and fibers the parenchyma (;>) of the pith-like tissue through which the bundles are distributed.— After Sachs. This lack of cambium means that stems living for sev- eral years do not increase in diameter, but become columnar DIFFERENTIATION OF TISSUES 291 shafts, as in the palm, rather than much elongated cones. It also means lack of ability to develop an extending branch system or to display more numerous leaves each year. The palm may be taken as a typical result of such a structure, with its columnar and unbranched trunk, and its foliage crown containing about the same number of leaves each year. The lack of regular arrangement of the bundles also prevents the outlining of a pith region or the organization of definite pith rays. The failure to increase in diameter also precludes the necessity of bark, with its protective cork constantly renewed, and its sloughing-off phloem. To sum up, the stems of the Monocotyledons are characterized by the vascular bundles not developing a cylinder or any regular arrangement, and by collateral and closed bundles, which do not permit increase in diameter, or a branch system, or increase in leaf display. 153. Pteridophyte steins. — The stems of Pteridophytes are quite different from those of Spermatophytes. While the large Club -mosses {Lyco- 2)odium) and Jsoetes usually have an apical group of meris- tem cells, as among the Seed- plants, the smaller Club-mosses {Selaginella), Ferns, and Horse- tails usually have a single api- cal cell, whose divisions give i n j-i, 11 „j!^u^„4-„«. Fig. 272. Diagram of tissueg in cross- rise to all the cells oi the stem. ^. , ", , , , „, . , section of stem of a icmiPteris), Generally also a dermatogen is showing two masses of scieren- not organized, and in such '^'jy^ '*'^>' ^«'"7^° and about ^ _ _ _ which are vascular bundles. — cases there is no true epidermis, chamberlain. the cortex developing the ex- ternal protective tissue. In the cortex there is usually an extensive development of stereome, in the form of scleren- chyma (Fig. 272), the stele furnishing little or none, and the vascular bundles not adding much to the rigidity, as they do in the Seed-plants. 292 PLANT STRUCTUEES In Equisetum and Isoefes the vascular bundles may be said to be collateral, as in the Seed-plants, but the charac- teristic Pteridophyte type is very different. In fact, the vascular masses can hardly be compared with the bundles of the Seed-plants, although they are called bundles for convenience. In the stele one or more of these bundles are organized (Fig. 272), the tracheary vessels (xylem) being in the center and completely invested by the sieve vessels Fig. 273. Cross-section of concentric vascular bundle of a fern (Pleris): the single row of shaded cells investing the others is the bundle sheath; the large and heavy- walled cells within constitute the xylem; and between the xylem and the bundle shealh is the phloem. — Chamberlain. (phloem). This is called the concentric bundle (Fig. 273), as distinguished from the collateral bundles of Seed-plants, and is characteristic of Pteridophyte stems. DIFFERENTIATION OF TISSUES 293 154. Roots. — True roots appear only in connection with the vascular plants (Pterido2)hytes and Spermatophytes) ; P P Pig. 274. Section through root- tip of P/eris: the cell with a nucleus is the single apical cell, which in front has cut off cells which organize the root-cap. — CUAMBEULAIN. and in all of them the structure is essential- ly the same, and quite d liferent from stem structure. A single apical cell (in most Pteridophytes) (Fig. 274) or an apical group (in Spermato- phytes) usually gives rise to the three em- bryonic regions — der- raatogen, periblem, and plerome (Fig. 275). A fourth re- gion, however, pecul- iar to root, is usually added. The a,pical cell or group cuts off a tissue in front of itself (Fig. 274), known as the cali/p- trogen, or " cap producer," for it organizes the root-cap, which protects the delicate moristem of the growing })oiiit. 37 Fio. 275. A longitudinal section through the root- tip of spiderwort, showing the plerome { pl), surrounded by the periblem ( p), outside of periblem the epidermis (